Were you successful with your upgrade?
On my side I have a similar system: M3970 fitted with a i7-3770 - 16 GB and a PS upgraded to 600W.
I tried two GPUs: EVGA GeForce GTX 960 and ZOTAC GeForce GTX 750Ti and I was unsuccessful with both cards.
Called EVGA Technical support and he mentioned that American Megatrend (AMI) Bios was not compatible with the GTX 960 GPU and that I should use a card of the 600 Series only. I have the 960 board in hand and the seller does not want to take it back even though my system was meeting the minimum requirements.
As the i7-3770 is still a great processor I really want to give it a second life!
I am looking for cost effective solutions, has someone been successful in upgrading the HD Radeon 7400 Series to a decent gaming GPU?