abacab
I am very happy with my 3rd gen Intel Core i3.  It runs at 3.4Ghz with two cores and 4 threads.  It has never held me back from anything I have tried to do.  When I built it I thought I could eventually swap in a i5 or i7 with same socket down the road, if needed.  But have never needed to!  It has been running great for over two years! 
 
I would probably have more performance headroom with an i5 or i7, but I never hit 100% CPU with what I have.  I mostly work with softsynth VSTi's and can load and run as many as my RAM will hold. 
 
The truth is most i3's are sold with budget systems, so everybody assumes that they are crippled.  But that is mostly due to the quality of the overall components used in off-the-shelf computers.  If you build your own and select quality parts, this little beauty can be a real sleeper!  Plenty of bang for the buck!!!
 
I have 8GB DDR3-1600 RAM and run Windows, Sonar, and all VSTs from an SSD.

Thanks a lot for your answer. Tell me how much plugins your i3 can handle comfortably if it depends on it.

Dust3R_FiLeS
 
Thanks a lot for your answer. Tell me how much plugins your i3 can handle comfortably if it depends on it.

There really is not a direct way to answer "how many plugins?", as they are all different.
 
Some are very CPU intensive (virtual analog synth), while others are heavy memory hogs (samplers), and some are a little of both. 
 
So, out of curiosity I ran a little test, creating a few fake benchmarks.  I used a 4 bar MIDI pattern in a loop, copied to all of the tracks, so that all tracks played back in unison.  Normally I rarely run into trouble, because I like simple projects!  These examples exceed how I would actually work.
 
Diva - https://www.u-he.com/cms/diva  I know u-he Diva is very CPU demanding, so I loaded it up until Sonar's audio engine dropped out.  I used the "divine" accuracy setting, which is the highest demand setting.  I got 5 instances of Diva running comfortably using about 65% CPU and 2.0GB of RAM.  With a 6th instance running, Sonar's audio engine dropped out intermittently, and I could tell Sonar was struggling to keep up.
 
NI Kontakt Player 5 - https://www.native-instruments.com/en/products/komplete/samplers/kontakt-5-player/  This is purely a sample playback engine.  I got 24 instances running at 30% CPU and 4GB of RAM.  I could have added more but got bored. so I stopped. 
 
Cakewalk Rapture Session - This is a hybrid synth/sample playback engine.  With 24 instances loaded, I ran at 60% CPU and 4GB RAM.  I am not sure what the limit would be here, since Rapture Session uses more CPU than Kontakt, but I probably could have added a few more tracks.
 
For an audio project with 16 tracks and FX plugs on most tracks, Sonar runs at 25-30% CPU and doesn't skip a beat.
 
Not to shabby for the "lowly" i3. 

Thanks to everybody who gave me an answer. Every answer is very helpful for me.

Dust3R_FiLeS

BobF
My previous machine was an i5.  CPU power didn't hold me back.

Thank you for your answer. Could you tell me what model of i5 did you have?


 

i5-3350P, IIRC.