I'm in a similar boat as the OP.
I have inherited an nForce4-based Gigabyte GA-K8NXP-SLI sporting an Opteron 170 (stock clock), 2 GB of RAM, and currently the world's hottest video card, the 8800 GTX (aka "Bakers' Delight"). PSU is a Seasonic 650W. It might be someone else's hand-me-downs, but for me this is a serious upgrade!
I loathe Nvidia's graphics, and my old box had an X800 Pro (AGP) in it, so after picking lots of brains and doing lots of reading, I settled on an ATi HD5750 based card of some denomination would give me the best back for the buck, with a bit of room to breathe into.
The problem is that not one, but three different brands of 5750 card refuse to work in my NF4 board. I tried an HIS one first, and like the OP all I got was full-speed fans, no beeps, no monitor activity. The board was perfectly seated, and the graphics power connected. I tried both PCIe x16 slots (the board is 2.0 spec), no joy, I tried booting with none, one and two monitors connected, nothing. I borrowed a 750W Antec and swapped that in, no joy. Take the card out, drop it into a friend's Core2Duo box (with a 380W PSU, I might add) and it comes up first go. Put it into a friend's Phenom II machine, it worked perfectly.
I tried a PowerColor card next, and apart from the PSU change which I didn't bother with, I got the exact same results -- nothing but fans.
So, I cop another restocking fee hit, and go for the Gigabyte GV-R575D5-1GD. You think a graphics card made by the same company that made the motherboard would work, right? Wrong. No change, full speed fans, no beeps, both slots tested.
The motherboard has the very last available firmware, so that's not an issue.
The only thing I can surmise is that there is a major conflict between the nForce4 and ATi's 57xx reference design.
Anyone else with any ideas?
Anyone recommend me a modern ATi graphics card that is known to work with nForce4 boards? I know a 3780 works, but what about the 4xxx series stuff?