http://www.tigerdirect.com
DFI Lanboy got excellent reviews, i believe the pentium IV socket is like 190 bucks. It has loads of features, and the new NFII chipset. However I have a few comments and suggestions to make before you go out and buy a new MB.
First, check your bios settings, make sure you have everything set correctly. Read through the MB and BIOS manual to verify proper settings.
2nd... Dump the FX card. I too had the FX 5600 and It has given me nothing but problems. Constant crashes, horrible benchmark scores and, worst of all... MOHAA looked like crap on it. So I went out and I bought an ATI 9800 Pro... If thats a little out of your price range, go to the ATI 9600 or 9700. For the past 3 generations Nvidia has had the lead for graphics cards cards, however ATI's new line makes Nvidia's products look like expensive plastic door stops. Don't believe me?
http://www.futuremark.com download 3dmark, and compare and contrast for your self.
3rd.
Make sure your DRAM settings are properly configured on you motherboard. If your not sure what settings they should be at, consult the motherboard manufacturer.
BTW, the most likely reason you cant adjuct your AGP settings is because your motherboard only supports a maximum of 4x AGP. The new cards have an 8x Potential, which requires an 8x AGP slot for maximum performance.
but... like I said...
ATI, and DFI lanboy will put you in good shape.