I'd go with 2 OCd 9800GTXs in SLI. Even 2 none OCd GTXs will kick a single 9800GX2's butt as any review of the GTX will tell you. The 9800GX2 only addresses 512MB of memory which is odd since they have 1GB. Anyone who says you can get 2 GX2s for the same price as 2 Ultras obviously hasn't seen what some places are selling the Ultra for--$429.
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.
At resolutions of 1920X1200 the 9800GTX in SLI runs Crysis faster than the 8800 Ultra in SLI and Anandtech says that, with 2 9800GTXs in SLI people can finally play the game at "very high". Personally I'd rather have faster cards with "only" 1GB of ram than slower cards with more ram since the extra ram isn't gonna mean diddly unless youre running massive resolutions on a giant monitor but, to each, his/her own. Try running a core clock of 770 and memory of 2300 like some of the factory OC'd 9800GTXs run with an 8800GTX.
That's what makes fast framerates if you have ample ram and, in almost all cases 1GB is more than enough. http://www.anandtech.com/video/showdoc.aspx?i=3275&p=4