Ive said it before, Ill say it again ... that is utterly useless for something like encoding. Most problems just arent regular enough for present GPUs. With the NV40, who knows.
MfA: I don't know, massivly parallel floating-point accurate DCT may be helpful. These new cards should have quite a powerful floating point processing unit. Not that I'm saying these cards will do that, but it's one way to speed up encoding.