cjrgreen: The PCI-Express version on that motherboard is 1.0a. Many modern PCI-Express cards will not work correctly in a 1.0a slot. We had another member of these forums get burned by this problem, and I still feel bad about having advised him to upgrade his graphics card, when his motherboard simply could not be made to work with a modern card despite many attempts.
AFnord: Me? Oh, those problems were from before I joined, so nevermind ;)
Anyway, the theory is that any PCI-E card should work on any PCI-E slot, but as cjrgreen stated, PCI-E 1.0a ports do have known issues with later cards. You might luck out and actually find one that works, but it is a risky prospect to go out and buy a new graphics card, considering the known compatibility issues.
It's pretty much a crapshoot. Maybe it will work, maybe it won't, but it's
your money you're spending to find out.
Anyway, with the BTX layout, just forget it. Nothing worth upgrading to will even fit.