Two 9800GTs perform equal to a GX2 or GTX280. You don't need the 1GB because it's not powerful enough to use all that RAM. (only the GTX or higher can use that much RAM)
So get the 9800GT SLi (512mb version) if you don't play to upgrade in the future, or get GTX260 now and another in the future.
If its the 1gb your worried about I think you can Sli a Palit 9800gt 1gb and a 512mb version. Not 100% sure though. But considering they're exactly the same other than Ram it should work. And so you'd have the 1gb as the primary and 512 as the second card. That way if it every needs to use more than 512 it can because of the way Sli works. But I'm sure you already know this
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