My new machine:
Asus P5K-E WiFi Intel P35 (Socket 775) PCI-Express DDR2 Motherboard
Seagate Barracuda 7200.11 320GB SATA-II 16MB Cache - OEM (ST3320613AS)
Corsair 4GB DDR2 XMS2 PC2-6400C5 TwinX (2x2GB)
Antec Nine Hundred Ultimate Gaming Case
Corsair HX 520W ATX2.2 Modular SLI Compliant PSU (CMPSU-520HXUK)
Gainward ATI Radeon HD 4870 512MB GDDR5 TV-Out/Dual DVI/HDMI (PCI-Express)
Intel Core 2 Duo E8500 LGA775 'Wolfdale' 3.16GHz (1333FSB)
Wont boot with the HX520 inside it - the fans do power up and all the lights come on but nothing on screen. The beep warning from a speaker I attached tells me it cant detect VGA. So I googled and saw someone mentioning a faulty psu being the problem. I replaced the PSU with an old 400w spare I had and it boots fine!
In both cases I definately had both cables attached to the motherboard and both pcie 6 pins attached to the graphics card. I have doube and triple checked this. After booting with the old psu i put the HX520 back in but no luck still.
I have emailed support both at the supplier (overclockers.co.uk) and Corsair but if anyone here can provide some ideas I would appreciate it a lot!