graphic card selection low-cost GF card v.s. professional FX card

I noticed GF9800GTX+ has 705GFLOPS, while GTX260 and FX4800 have the similar FLOPS (pls correct me if I am wrong…). If just for CUDA computing purpose without display output, what would be the major difference to use the three of them? GTX260 and FX4800 are both much more costly…

The GTX 260 has more memory and much faster memory than the 9800 GTX. In addition, the GTX 260 supports double precision arithmetic (8 times slower than single precision) and has twice as many registers per multiprocessor.

The FX4800 uses the same chip as the GTX 260, but with more memory running a little slower. Unless you need 1.5 GB of memory, you are better off with a GTX 260.