I just switched out an NVidia 7600GT (non-CUDA supported) with a NVIdia GT240 512 MB, DDR5 (96 CUDA cores). Not the greatest card, but not that expensive either and could be used with my older power supply. cpu: C2D E6700 2.66 GHz, 4GB RAM
A typical project of ~4 mins including 6 MP stills, 1440x1080 footage, some transitions, 5.1 surround sound rendered to .m2ts using Sony AVC codec with AC3 gave 31 mins with the original video card. New graphics card with CUDA gave under 26 mis. The 5 minute savings corresponds to a 16% savings in rendering time.
So, it seems that CUDA cards work and give significant time savings, at least under these conditions.