Hello,
for really flexible hardware shading, you need 8xxx or better (shader model 4). Then you can launch CUDA programs on your graphics card. If you look at the latest NVIDIA demos (human head), it's quite impressive. The next generation GPUs (Fermi) will be very interesting, because they can do
double precision arithmetic as well very fast, so wait until next year before upgrading ;-) .

Best regards,
   Michael