I currently Own a EVGA 7800GT CO(The factory overclocked Version), I would like to possibly go to SLI as this isnt quite cutting it in Vanguard. I can get a EVGA 7800GT for $130 to put in SLI mode. This is going to slow my Overclocked card down to its setting I believe. I cant seem to find another 7800 GT CO for under $200 so I really dont want to go this option I dont think.
Will this be faster then going with a single 7950 GT that I can get for around $200?
Which is going to give me the bigger bang for the buck?
The 7800 GT is a nice GPU but it is getting old. SLI is usually hardly something worth the money. I can see that you consider it for a price like that though. Performance wise two 7800 GTs are faster than a single 7950 IF the game can utilize SLI or the drivers do their job properly. If they don´t you have the same performance, as you have right now, maybe even worse. In addition you may need a new PSU.
I´d sell the 7800GT and go for a single GPU solution. The best option would be to wait for the R600 and mid-range Nvidia cards, then sell the 7800 and buy the fastest single slot card that fits your budged (maybe the 8800 GTS with 320 MB?).
One of the Graphic Card manufacturers has a step up program. Using that you could buy a 7950 now and upgrade quite cheap to a bigger one once they come around sometime next month.
I could understand someone buying a 7800 or 7950 if coming from something like a 6600, but in your case, upgrading to another DirectX 9 card seems like wasting money. Yet, in the end, it all boils down to how much you are willing to spend. 8)
Or even better save your money for a DX10 card. The 7800GT is a great card but if you're having trouble in a game you can easily turn settings down. Or overclock up. Now is a bad time to upgrade to another DX9 card, you would be kicking yourself later.