Same boat as you.
In one hand my whole computers needs upgrading for the future
My specs are
Opteron 165 duel core 1mb cache, @ 3ghz
2gb PC3200 G.skill HX 250mhz 2.5-3-3-5-1t
8800GTS 640mb
It may be outdated in computer terms pushing 3 years now for me, but it still plays every single game iv played at or near the highest details with the max AA/AF or 4xAA 8xAF but at 1280x1024 limited by my older VX922 LCD 19"
The only games i have a little trouble on are very few like crysis ect, but i dont really want to upgrade over 1 game.
My problem now tho is my VX922 died im getting the new monitor tomarow, and its a upgraded model still 19" but it has a 1680x1050 resolution, so im not sure how my pc will perform at that resolution i dont think itll be a problem tho.
Anyway to the point, i was also thinking of getting another 8800gts 640 and do SLI mainly cuss of the resolution upgrade on that monitor and for future games. I know for a fact the fps will jump about 20 maybe 30 on some games or more who knows. I havent even overclocked my GTS yet to play games. So i might get even more if i overclock the cards in SLI.
There is also the choice to sell the GTS 640 and use the money you were gonna buy the other one with and the money from the one you sold, and buy a whole new video card like a GTX 260 or 4870 which will get you a even bigger jump then SLI would.
I decited tho since i have only 2GB of memory and that my main problem atm is the lack of memory that my pc is slowing down when i alt tab games. Even wow when i play for a little bit then alt tab i see the memory usage is at 1800-1900 MB and my pc takes like 20 seconds to even open IE when i click on it. Same with other games like COD4 ect. With the higher resolution the memory usage will be even more so more memory is a higher priority for me.
I relise tho if i upgrade my memory and cpu to DDR2 and one of the new quadcore's that my fps will probly go up as much as the SLI video card would. Either way each upgrade is big lol.
For you tho i would sell your GTS and use the money you had and that money and buy a whole new video card. Its like this basicly sell your card for like $130, use that %150 you had and buy a 4870 or GTX 260 that are only like $290