Two 9800GTs in SLi is basically a 9800GX2, so it will be equal to or slightly superior to a GTX280. The GTX280 is in turn, superior to the GTX260-192/216, which in turn is equal to the 4870.
If your board is an SLi board, go for the 9800GTs in SLi. If it can also handle Xfire, consider 2x4850s or 2x4870s. Just remember that SLi is more bang for your buck now, but it is harder to upgrade in the future.