Maybe add more RAM, but I was doing all that stuff on 512 mb RAM before I upgraded to 2.5 gigs. First of all, is your system optimized well? Also, work on the latency settings as well as the other audio settings in Sonar to improve your setup.
As far as a VSTi which does drums, pianos, rhodes, B3, etc. all in one, there are those ROMplers which do that, but in my experience the quality is pretty poor as opposed to getting VSTis for each specific instrument.
For pianos, check out the new TruePianos VSTi. It's SUPER low on CPU, gives great realism, and has a lot of playability from a pianist's perspective. Other than that, most of the other good piano solutions out there are multi-gig libraries which you'll have to stream off your HD.
For B3 organs, there is Native Instrument's B4. I use Organized Trio, which is really comparable, and free.
For rhodes, MrRay73 (also free) is definitely one of the best. Not to mentioned that it's physically modelled, so it gets great playability, but also, it sounds and feels more realistic than most of the wavetable rhodes VSTis out there as well.
For drums, there are a bunch of great solutions. Addictive Drums seems pretty cool, with its mic mixing capabilities and the numerous drums which you can build your custom drumset with. I can't speak for BFD or DFH since I haven't used either apart from the demos, but I've heard good results from both.
My top choice for drums, however, is the NSkit7, which is a 5 DVD library (which is amazingly priced somewhere around $100 USD). As well as getting numerous choices for kicks and snares and whatnot, you also can play it with brushes, mallets, hands, as well as sticks. Not to mention each drum is sampled with left/right hands (with automatic cycling if you want), and lots of different articulations--stick shots, rims (or as near to the rim as you want in some cases), hihat openness (the hihats alone are 5 gigs), the kick closed or open (whether the beater bounces off the kick or sticks there), cymbal grabs, etc. There's also lots of other percussion goodies that come along, a cowbell (played with sticks, mallets, etc.), a tambourine, and the best conga set EVER--quinto, requinto, tumba, and tumbadore--with real conga techniques in the samples.
Anyway, I think I've plugged enough products.
I'm not affiliated with any of these companies, just a fellow consumer...