ASRock 775Dual-vsta & 8800gt no boot or post

I have just bought an 8800gt xfx as my old 6600gt died. But the trouble is, the system won't post or boot with this new card in. I should probably point out that the old card was AGP and the new card is PCI-E. Does any one have a similar set up or have any idea why this may not be working?

I posted this previously but gave the wrong board details (half asleep).

Thanks. Have changed the setting to PCI-E already and have a 550w power supply so should be sufficient.