I remember typing in some thread that perhaps that series of Gigabyte board had issues with GF 4 cards. I've looked at a lot of different forums over the past few days,
http://www.viaarena.com and anandtech to name a couple and this seems to be a common issue.
This isn't a VIA chipset issue, since lots of people have got the GF 4 cards to work, but rather it seems to be an issue with the MBs themselves. I personally think the issue is related to power or possibly a flaw in the GF 4 design itself that keeps it from working on many boards.
My advise would be to call Gigabyte and talk to them and if the problem can't be solved within a day or two, RMA the board. It's not worth messing with it, trying powersupply after powersupply and flashing the bios over and over hoping to get it to work.
My problems all went away when I switched to this Asus A7N266-C board, which uses nVidia's nForce 415D chipset. So far I haven't had a bit of trouble with it (though I haven't yet reinstalled everything I had on my old computer). I have messed around with AH offline and everything works perfectly as far as I can tell. This board doesn't have USB 2 or Raid, but I didn't need a Raid controller and I can always go out and buy a USB 2 controller if I need one.
The only thing with the nForce boards is they are kind of picky on memory. You can only use 2 double sided sticks of RAM in slots 1 and 2. Slot 3 must be a single sided DIMM. I just put in 2 x 256 MB Corsair PC 2100 sticks in slots one and two and all works great. You can use 2 x 512 MB sticks if you want though.