From one GTS 320 owner to another please take my advice and DO NOT SLI your cards. When you SLI, you will NOT be doubling the amount of memory available to the cards. Therefore, you still still be limited to only 320 Mb. I'm almost certain that every single game you try to play at that res (even with no AA) will immediately crash. 512 Mb would be the absolute minimum amount of video memory i would even consider at that res.
If it were me, i'd game on your current monitor until at least the end of the month, when the 4xxx series comes out, then decide what card to get. If the new cards rock, get one of them, if not, they'll at least shake up the price war so you can sell your GTS, buy a 8800 GTX or 9800GX2 and then step up in July or something.
just my 2 cents.