Well, two 9800 GTX+ cards in SLi would generally be faster than a 280GTX and slightly faster than a 9800GX2. Of course the 9800GX2 is going rather fast so supplies are limited as they say 😀. Anyway, compare charts to see the difference between the 9800GX2, which is basically two 9800 GTX cards in SLi, and the 280 to give you an idea of how two 9800GTX+ cards will fare in SLi. While in most cases the SLi cards would be faster, there are a few instances where SLi does not scale well and the 280 comes out on top. The 280 also comes out on top at high resolutions and settings where the memory is taxed. The extra memory on the 280 will also shine in future games while the 512MB on the 9800 GTX+ may be a limiting factor in the future.
You need to take all of that into consideration before you choose. I say that if you can afford it though get a 280 GTX or 260 now and then another one latter to run in SLi. If you really need the power of two 9800 GTX+ s now then buy those and run them in SLi.