Forget about what addt'l system memory a card supports, it's a 128MB card which is not sufficient for future gaming and limiting for present gaming (no high or max quality, which typically requires 256MB minimum on-card for best results).
You can't spend less than $120 and get a gaming card that is a reasonable upgrade from 6600GT. There are some with a little better performance per same game, BUT to buy today looking ahead several months at other newer games you'd have a lower performance relative to the games than you did with the 6600GT in the past.
You can get a card that supports some newer features for less but the overall performance with those features active/enabled will be little to no better than the 6600GT with what it does support.
The best bang for the buck on budget ~ $100 cards is probably either a radeon X800GTO (unlocked to 12-16 pipes if yours allows it) or an nVidia 7600GTS which tends to be a GT level performance card when overclocked.
If you are building a new PCI-Express system for gaming I suggest delaying until you have more $ to put towards the video card. Faster CPU and memory will do no practical good until the video card is at the next level, you might as well just reserve the old system for gaming in the interim.
trust me the extra $20-$30 bucks is worth it in the long run, and the cards are way more future proof than a 7300gt or a 6600gt.
I you buy a 6600 today, by next summer you'll wanna shoot yourself for not buying a better card. lol