I would upgrade for the future, get yourself a 512MB card. Because in the next few months nVidia will release their 8 series product, and in two days ATi is releasing their new product, dubbed the X1900 series. The X1900 has a huge edge over the 7800GTX 512. The X1900XTX will feature GDDR4 RAM, 48 pixel shaders (pipelines?) and it will also boast a 1.7 GHz memory clock. Needless to say it's a GTX killer!
Or, if you want to, buy yourself a 7800GTX, then, when Christmas rolls around, get another one, you'll be rolling with the best of them if you get a 7800GTX in SLI!
Overall, ditch the 6600GTs. Get soemthign new, that way you won't have to upgrade to more expensive parts in the future.