HD4870 and GTX260 crash system, HD2350 doesn't. Why?

Every time I try to install either an HD4870 or GTX260 on my system (spec below) the system crashes just as it loads the drivers. Strangely the HD2350 and HD2480 cards I've tried work no problem. The system also runs no problem when using onboard graphics.

At first I thought it was the graphics card so tried the various cards as above, but when the problem was still there after trying the HD4870 and GTX260 I thought it must be a motherboard problem. However, i've now had 3(!) new motherboards and the problem remains, so that rules the motherboard out I think, unless it's some obscure compatibly issue. I know all the cards are OK as I've taken them out of other systems where they work fine.

*All drivers/bios up to date
*Multiple clean installs of drivers (including core drivers with no CCC for example. Also tried older and newer drivers)
*Multiple clean install of the OS
*No system problems with any other components as far as I can tell. (I've ran memtest and CPU stress tests)

OS: W7x64 SP1
Mobo: ASUS P8H67-M (B3 revision)
CPU: Intel Core i5-2500K 3.30GHz (Sandybridge)
RAM: OCZ 4gb DDR3
PSU: Corsair HX520 (this should run the system no problem)

dmp file attached. (When using the HD4870)

Have you tried testing a new power supply? If older/onboard graphics works no problem, your PSU might not be handling the power from the newer GPUs properly.

Yes i've tried a 700w PSU as well.

Problem solved, I had a run of 3(!) faulty motherboards. I've moved to a different model (and manufacturer) now and all is well.

I won't buy ASUS again as 3 faulty chips is a joke!