i have an 8800GT, and i play at 1680 x 1050, and i could max out all the games i have tried. The only games i couldnt max out is Crysis, and Stalker Clear Sky.
to SLI, you would have to have an nForce Chipset SLI compatible motherboard, 2 SLI compatible graphics cards, and an Power supply that will supply enough Watts and Amps.
i saw some benchmarks where 2 8800GT SLI will beat an HD4850, so i am assuming 2 8800GT SLI will beat an 9800GTX+
EDIT - But why SLI 2 9800GT? they will end up costing more than the 9800GTX+, why not get an 9800GTX+ now, and SLI them later when you get more money?