I bought a Gigabyte Gx2 yesterday, took it home, no post, nada, nothing.
Tried it in my 2nd PC, same issue (the card was getting enough power etc but the motherboard did not recognise the card).

Took it back to the place of purchase and had it replaced (glad that they also could replicate the problem).

Now I have another one sitting in my bag here at work.. another 6 - 7 hours before i get home to try it out.. It will be replacing a 8800gtx, since I am on a P35 Mobo and not wanting to get SLI, this is the best option for now, and I have a work colleague wanting to buy my 8800, so all up, should be a cheap upgrade.