Obviously if the OP doesn't mind spending too much, the 9800GX2 is a "good" choice.
I'd rather go with 2 x 8800GTX, instead of 2 x 9800GTX. Actually that depends on the resolution, but the extra memory on the 8800GTX should help in some games. You can find the 8800GTX under $300 and 2 of them should beat out a 9800GX2. There are also less issues with drivers and such.
Or you could go with Tri-SLI with 3 8800GTXs. It's cheaper than 2 9800GX2s, that's about it.