or doing alot of folding apps the performance diff is too large to ignore.
Yeah the GTX480 just sucks at GPGPU;http://hothardware.com/Articles/NVIDIA-GeForce-GTX-480-GF100-Has-Landed/?page=15
There really aren't many 'folding apps', most other clients have updated to a standard that is far better than F@H which is simply crippled until they update the core. The HD5970 would outperform the GTX480 if it was given equal access to LDS.
I would say for anything you use regularly that is coded in CUDA go with the GTX480, for more OpenCL and Direct Compute stuff that's more universal, then both will perform pretty closely depending on their requirements (although single precision is actually about 4:1 on the HD5970, but it's hard to exploit as you can see from the Sandra results where Fermi and the HD5970 are neck & neck).