I was in a similar spot, with a 320MB GTS and a 22" LCD, and a desire for fsaa in most games. Oblivion required some compromises, but was OK. Crysis was one the one to make me upgrade as the 320MB GTS is pretty bad at that game without dropping the res alot. While SLI would scale well in most games, 16x10 with 4xaa is getting too high for that 320MB card in other games. ANyway, besdies that, hate to steer you clear of SLI as it does scale very well in most games now. Two of your cards probably beats the 8800 GTX most of the time until you get to settings where the memory limitation kicks in.
If your brother needs a card and games at a lower res, it could be win/win for both of you. An 8800GT or 8800GTS G92 for $160-200 AR are fine upgrades from that card that in certain games will allow for better 16x10 + fsaa gameplay. SLI 320MB GTS will beat these cards in many games, but again, your res and fsaa make me leary of that route as I was at the time I made this choice.
Have a look at this GF8 comparison and specifically the 16x10 green bars. You will see just how nice a step up the G92's are from your current card for Crysis. Not a huge leap in some of the other games. Oblivion would also be a nice step up. http://www.legionhardware.com/document.php?id=710&p=2