I have an MSI KT4V MS-6712 motherboard with Athlon XP 2400+ CPU and Radeon 9700. Last year I tried to upgrade from Radeon 9700 to Geforce 7800 GS, but Windows XP refused to start properly with the new card. I tried different things but always got black screen right before the first Windows XP loading screen. I gave my computer and the card to the company who sold it to me. They installed the Geforce 7800 GS on one of their computers without any problems, but couldn't get it to work on my system. They weren't certain what the problem was, but they mentioned that they had some kind of problems when installing nVIDIA drivers. To my great luck they accepted the card back for a refund. I myself suspect that the problem is related to the AGP port of the motherboard. According to the motherboard manual MSI KT4V MS-6712 supports 4x and even 8x AGP, but in my case AGP acceleration is for some reason disabled in Windows XP even though it is enabled in BIOS. Catalyst drivers always switches AGP to 1x after I try to set it higher and restart Windows.
Will I have the same problem with Radeon X1950 Pro as I had with Geforce 7800 GS or was the vendor right and the problem really was with nVIDIA drivers and any new AGP card produced by ATI should work fine on my system? The strange thing is that my Radeon 9700 card works just fine and runs game very nicely even at 1x AGP. I guess if Radeon 9700 were only 4x/8x AGP cards, then the problem was with nVIDIA drivers and an X1950 Pro should work fine too. But if Radeon 9700 is the kind of card that can run perfectly fine even in 1x AGP, while the newer cards cannot, I guess I won't take the risk and just start saving money for a completely new PCI-E system.
Btw, I have a 400W PSU, which is quite enough for a Geforce 7800 GS, i.e. PSU is not the problem.
Then I guess an X1950 Pro might not function properly on my system. BIOS upgrade might help but it could damage the motherboard permanently and I cannot RMA it anymore because the warranty period has ended long time ago. Fortunately, Radeon 9700 is still an adequate card. It even manages to run some of the newest games quite nicely, even at 1x AGP. I'll probably wait for R600.
Some of the newer bridged 7x00 cards need an option set in the BIOS to work on some boards. I had a problem getting the 7800 and 7600 GT to work on my ASROCK Dual-SATA2 until I found this setting, forget what it is though...
I don't think they made an AthlonXP motherboard that wasn't AGP 4x capable, and all AGP 8x cards will work on a 4x board.
But regardless, it looks like you have 8x anyhoo. From the MSI website: