Since you only have one smallish monitor you might want to set yourself up for just dual SLI. So if you want it, that means that you just need 2x 6 pin + 2x 8 pin to be able to run two of the biggest cards. No point in going Tri SLI.
If you're looking for top of the range, here are the 4 top cards.
HD 4870 512/1gb
HD 4870 X2 2gb
The above are quick but the 4870 won't SLI and the 4870 X2 doesn't need to as it already is two GPUs on a card.
Advantage - Direct X 10.1 - Forward compatible with Windows 7, Runs anti aliasing better than the Nvidia cards.
GTX 260 - 192
GTX 260 - 216
GTX 280 - 240
Advantage - Can SLI, also can do a whole bunch of potentially interesting things in the future like SLI and accellerate your photoshop programs etc. Also can be found with a deep discount and they come with Far Cry 2 at the mommen.
My suggestion is that any one of these cards will be more than fast enough for your monitor size so just select the best balance between price/features. The Nvidia ones have a deep discount/MIR so they may be better value for you.
Tease.
