Unless you have a 64 bit OS, 6GB of ram would be pointless as Window's would only recognize 3.2GB. I have 4GB on Vista ultimate, and even when doing heavy video editing and gaming, i have yet to use more than 70%.

In the end it all comes down to your preferences, as i am only advising. The 7950 series is by no means lacking, but given the current standard they are all but near-obsolete. Especially factoring in their DX10 incompatibility. If you looking to just "add a little power," SlI'ing another 7950 will give you just that. But if you are looking to put some serious horsepower under the hood (not to mention future proofing your system) then a 8800Gt or GTS would be it.