Dead(ish) Mac Pro

Hey all mad dilema. Bought a Mac Pro Original 06 with ver2 mobo and looks like some fool had dual boot with windows and has completely obliterated the mac hdd and now the thing wont boot any OS i try. A friend let me borrow an old snow leopard thinking that could restore the HDD but no luck. On my own mac pro 08 i did however wipe an HDD and clean installed then put that HDD in the 06, no boot screen but waited a few minutes and the welcome video plays. Have sound but no picture. Was a GTX7800 original card initially so bought a GT120 Nvidia incase the card was duff but same storey. I then went all out and discovered people had mac os on pc so dug deeper and found a bootloader which allows me to boot my snow leopard hdd but upon start under About This Mac my bootrom is not what it should be. ANy way to reverse this or rescue the poor old dog