Stand alone soft synths ...

LOTS of them.  depends on how much money you have!  range anywhere from $20 to $4000 per package!

the best for the money, IMO, is Kontakt.  it's not "cheap" but it sounds great and delivers great quality.  Costs about $400 new without educational discount.

it depends on what sounds you're looking for as well.  if you want orchestra sounds, consider GPO.  ~$180, IIRC.

what kind of sounds are you looking for and what budget are you looking at?

Any of the ones Beagle suggested would work fine. 

With software synths, I would imagine the biggest issues would be switching sounds quickly and stability of the whole platform. 

If you needed to switch from piano to strings in the middle of a song, that could be a problem unless you know some sort of short cut.  For me, switching is not a quick thing.... but in  a studio it doesn't factor in.

Stability of the platform..... you are running  in a windows based environment with soft synths.  Crashes and glitches happen at the most in-opportune moments. And nothing says in-opportune like a crash in the middle of a song on stage. 

Personally, I would visit a music store and have a look around at the hardware synths and the keyboards they offer. I think you will find something in the price range you're looking that eliminates both of these issues and still offers good sound quality.

Hi Salleke -
yes, that route will work using standalone softsysnths and a computer for your MIDI output, BUT there could be many complications with that kind of set up.  I have considered doing that myself since I have Kontakt and several other good softsynths and have considered setting up a laptop and running my keyboard thru the laptop for softsynths.  but it scares me to try it!  Jim Roseberry does this live and has great success at it (and Jim is a very successful musician and computer builder on these forums).

the main thing you need to consider doing something like that is that the computer needs to be good solid performer.  I'd go with win7x64 and at least 6 to 8G of RAM with a dual or quad core.  but your soundcard is going to be more important than that!  you need a good solid "recording" soundcard with solid low latency drivers.  if you don't have that then you'll never be able to perform live with softsynths.

another consideration is that if you want to go this route, it would work better if you have software which will allow you to run the softsynths in sevearl saved configurations with flexible I/O.  Catible is a software program that loads multiple softsynths and allows flexible routing and saved configurations for performing live with softsynths.

considering all this - if you don't have most of it already then it's going to cost you a lot more to do this type of setup than it would to buy a hardware module.  the trade off is that the softsynths like Kontakt have MUCH better sounds (espeically strings) than any of the hardware modules out there because they're mostly sampled, not mostly generated. 

another route you might want to consider is Band In a Box.  BIAB uses "real tracks" and MIDI I/O and they sound really good.  you still have to have a softsynth for MIDI (or hardware synth) but the mix of the real tracks and MIDI make a great combo for backing tracks.  BIAB also has a "composer" feature which is like a playset type of thing that allows you to run songs in a playset.  I haven't used that feature myself, but I know a lot of folks who use BIAB run it this way.

there are a lot of folks who use BIAB as backing for their live performances.  however, a decent computer and a good "recording" type soundcard is still essential for that type of setup.

Personally I have been going a slightly different route.  I recently bought a guitar pedal "looper."  I create backing tracks in BIAB and in Cakewalk, then I record them onto the looper in a memory location.  with the press of a button on the foot pedal, I start the backing track and I play along with my MO6 and sing.  it's working out pretty well.  the looper cost me about $170 and I'm using BIAB real tracks and Kontakt in Sonar to create the wave file, recording that on the looper and I have a rock solid backing track system.