|Your motherboard manual states 8X AGP (1.5v only) but that only refers to the slot.|
AGP Bus Tutorial
From the above: "AGP connectors and slots are physically keyed to specify the signalling voltages the device is capable of operating at. AGP specifies two keys, a 3.3V key and a 1.5V key. Unfortunately they added a third voltage, 0.8V. If a device operates at 0.8V it uses the 1.5V key and must be tolerant of 1.5V signalling, but not necessarily capable of operating at 1.5V. A key is manifested as a raised area in a female connector or slot and a gap or absence of pins in a male connector. The keys physically prevent an electrically incompatible AGP video card from being inserted into an AGP slot."
AGP compatibility for sticklers
From the above site, here's the easy answer: "If an AGP card fits in an AGP expansion slot then they are compatible."
I don't understand why you'd buy an X1950XT in the 1st place? If you're gonna fork out that kinda money (I'm sure it ain't cheap), you should have gotten the best card ever made for AGP - the Radeon HD 3850. It doesn't get any better than that (& never will).