AGP is a bitold hat now but come 4 speeds : normal,x2 (times 2), x4, x8
click here for more info.
So far as I am aware, each cards has its slightly different fitting so you cannot fit the wrong one, but to be perferctly sure you need to check your Mobo manual. P4P800-SE has I AGPX8 slot (which is 1.5 volt), but, as long as you are using An AGPx* card, you should have no problem. However, I'm wondering if you are looking in the wrong direction. If you have the same problem with a new mobo, and the graphiocs card fits properly, next step must be to check your monitor and its cable.