Its OK, not the best and it is a little loud. For anything from a GF3 or radeon 8500 and down it will probably be slightly better then the stock heatsink fan, I dont think it would be much better if at all for GF4 Ti cards or 9700s.
The best heatsink for a vid card is a 1u (rackmount) HS but that will require you to epoxy it (arctic alumina epoxy) to the GPU. I believe thermaltake also makes a HS called the tiger or something which has a lower RPM fan but would probably be better then the Corb but I dont know about that for sure.
I just searched all of the ads on the overclockers.com front page and found a site that sells them, Ive never ordered from them but since they advertise on here I would imagine they are a good company (although, of course, overclockers.com assumes no liability and all of that) I wouldnt hesitate to order from them.