CPU - Incredible S CPU is, at most, 5% faster than the CPU in the original Galaxy S. This is in terms of common applications. There are some features that is has (128-bit SIMD instruction set, for example) that it shares with the Cortex A9 that the A8 (used in the Galaxy S) doesn't have. For everyday applications, the Incredible S CPU is the winner, if one has to be picked.
RAM - 768MB (Incredible S) versus 384MB (Galaxy S, 512 if you count the separate 128 dedicated to the GPU).
GPU - Incredible S has a solid GPU, but the Galaxy S has on that is roughly 50% faster, and has 128MB of dedicated RAM.
If you're a gamer (and I don't mean Farmville), get the Galaxy S. Otherwise, get the Incredible S.