I have the Mac Pro Model 1.1 2ghz.
What sort of options do I have for a custom flashed card? I'm quite interested in the ZOTAC GeForce 8800GT AMP Edition.
Would I need to flash the card with a EFI32 image?
You need an EFI32 ROM and some way of actually editing that ROM so the card retains its BIOS clocks. When running in OS X you will only get the default clocks.