Question about samples

I am returning to home recording after a few years off with a new PC. I need a synth for virtual instruments. I will want some warm Mellotronish synth sounds, but also some realistic classical instruments that I would sequence via Midi. When I last was recording, internal synths were still the rage. I assume today all virtual instruments are companies that sell gigabytes of samples which one uses Sonar to sequence, rather than hardware synths? I have a small three octave keyboard I'll use as controller. Would I be researching companies that sell samples with the PC acting as a sampler that changes keys of samples with one's controller? Wouldn't that require all digital samples to be in the same key if I'm going to be playing them on a controller in different keys?

There are plenty of "kinds" of soft synths that use your PC to generate sounds.  Many our Virtual Analogue, software versions of Moog etc. (Zeta as above).   Most are sample-playback synths, that use a prefab library of synth and  acoustic/electric instrument sounds (Rapture and DimPro are Cake's best examples).  There are few "samplers" that let you actually sample a sound internally and work on it.
 
I'm not sure about your question about Keys and chords.  Most shorter keyboards let you increment the actual keyboard by octave up and down the range.  So tho you can't play all 88 piano notes at once, you can play every note, which is an answer to your question, I think.  Kontakt (probably the standard sample-playback synth [tho you can certainly stick your own samples in it] will work with keys/chords, although I think that is only in conjunction with their hardware.  I just got a Maschine Jam hardware controller and you can play the pads like a keyboard and pick only pads that are in your key so it will only play those notes.  "Look ma, no bad notes."  "Well, Johnny, you can't play jazz then."
 
I hope this helps.  If not, refine your question and ask again.  Plenty of people with better answers here.

DanielVogel
I am returning to home recording after a few years off with a new PC. I need a synth for virtual instruments. I will want some warm Mellotronish synth sounds, but also some realistic classical instruments that I would sequence via Midi. When I last was recording, internal synths were still the rage. I assume today all virtual instruments are companies that sell gigabytes of samples which one uses Sonar to sequence, rather than hardware synths?

There are probably more analogue and digital hardware synths on the market now than at any previous time. And more software synthesisers as well.

They hardware is mostly much more affordable than it used to be a couple fo decades ago as well. What has pretty much disappeared is hardware "General MIDI" specification sound modules and keyboards which simply had astandard set of "instruments" such as "piano", "bass", "oboe" etc. of usually pretty unrealistic quality, and intended to ensure that MIDI files for the music in e.g. games sounded roughly the same no matter the platform they were played on. Nowadays software-based samplers do a much better job of impersonating the "real thing".

DanielVogel
I have a small three octave keyboard I'll use as controller. Would I be researching companies that sell samples with the PC acting as a sampler that changes keys of samples with one's controller? Wouldn't that require all digital samples to be in the same key if I'm going to be playing them on a controller in different keys?

Samples of instruments that are intended to play as an instrument rather than audio loops generally load into a software sampler instrument. Rapture, Rapture Pro and Dimension Pro are the Cakewalk ones, there are many others from e.g. Native Instruments and IK Multimedia to name but two. The sample instruments supplied by the sampler manufacturers generally need no user intervention to ge them into the right key or pitch, the software instrument takes care of that. Providers of sample libraries usually either supply a suitable player for their product or state which sampler(s) their product works with.

Pre-recorded audio loops (short passages of up to a few bars, some people build entire compositions based round them) may need re-pitching to get into the desired key, but pretty much every DAW can handle doing that. Though if taken too far from original pitch loops can start to sound a bit strange.

In short, don't over-think this. Most of the hard work has already been done by hardware and software instrument designers and sample creators.

For Mellotron consider G-Media M-Tron (Pro).

If you are buying already, I think the top version of Sonar (Platinum) still comes with Addictive Drums 2 which is a great drum program for acoustic/real drums - it comes with three kits and lots of midi loops and you can obviously edit the loops in Sonar.

The programs we're talking about will occasionally work as stand-alone but mostly you'd open them within Sonar, they're called plugins or virtual instruments or sometimes VSTi (Virtual Studio Technology instruments, a standard devised by Steinberg).

Kontakt is pretty much the standard for software samplers these days. You can buy it separately or as part of a bundle called Komplete which comes with a bunch of other sound expansions and other software synthesizers and fx. Currently the manufacturer, called Native Instruments, is holding a sale I think.

Altogether you'd be looking at quite a lot of money, around 600 for the top Sonar version and another time something like that for Komplete, even with the sale. M Tron Pro is cheaper but it also depends on how many expansions you want with it. With Sonar comes a basic version of Melodyne which is an awesome vocal editor (can also be used for other stuff) that you may want to upgrade for another 200 euros or so. It's really easy to spend your money these days :)

But still a lot cheaper than buying a Motif or Triton ten/fifteen years ago. And obviously you can go step by step, you could get a simpler version of Sonar, or you could get the top version but wait with buying Komplete or Kontakt (because it already comes with loads of stuff).

Yes, an upgraded new version of SONAR can solve a lot of your headaches.  Cake has a upgrade Sale going on now for early SONAR X series.  Call sales and see if they can help you.
 
SONAR comes in various strengths, although the engine is the same the goodies aren't.  Think of it like a loaded car.  You pay more but you get more.  The top level of SONAR gives you a built-in drum machine (see above), built-in synths including Dimension Pro (a sample playback synth) and Rapture (same engine but the included samples are more synth-like rather than acoustic).  There is a bunch of other synths and effects and reverbs included, so you don't really need anything more than SONAR.  It is a good way to get a running start.
 
Then later you can add the Native instrument stuff, and there is cheaper ways to do it.  I bought a NI Kontakt library, Heavyocity, on sale which could use the "free" Kontakt player.  I upgraded to a paid Kontakt for like $200 during their Christmas sale.  The next summer or so I bought Reaktor for another $200.  I bought the hardware Maschine on sale which came with more synths and can now upgrade to their (un) Komplete collection for another $200 or their Ultimate for $400 (NI has 1/2 price summer and Christmas sales every year).  Like Cakewalk, they want to keep you in the family.  I don't know how much money I've saved this way but I spread the pain over 4-5 years and have NI pianos and moog and a bunch of other stuff I didn't want but now like.

Only Platinum comes with all those synths included! Though "professional still has a decent subset". Be sure to check the feature comparison page. I don't use all Sonar stuff all the time but if you're starting from scratch, Platinum, while the most money, is a very competitive package.

Thanks all for your baker's dozen replies! I'll start with Platinum and that will keep me mighty busy for awhile and too poor to buy anything else for a while. I worry I won't be able to figure out the whole shebang - so many plugins, I'll be turning into a Borg. Are they relatively easy to learn for a pea brain like me (I'm a psychologist by trade not a NASA Hidden Figure)? Will I have time to create?