ask for suggestions on video card for CUDA applications

I am a newbie to CUDA
i was attracted by CUDA’s high performance, and believe CUDA might help alot on my current project, which involves tremendous FFTs
to start with, i have to purchase a video card supports CUDA first.
i dont play game, and the major computation for me is FFT, Hilbert Transform, interpolation for image reconstruction, and may be a simple 500500500 solid volume 3D display after it is reconstructed
the budget is <1000, thus a video card is preferred,
so, any suggestion? 280GTX or 9800?
or can i put two or 3 cards for a SLI mode computation?

SLI is not supported (not automatically, but you can partition your computation manually). Go with GTX280 (or 260) since it has some new features over the old generation.