Video card compatability?

I am a little confused on the video card agp compatability. My Mobo's user manual says it has an AGP 32-bit 1.5v 66mhz 4x/8x Bus slot. When looking around for a new card I find all the 1.5v agp are 64-bit or even 128-bit. Physicly the bus's are the same. How does this work out. Would these cards even work properly. Getting kind of confused. Can some one shed some light on the subject for me? I want to make sure I get the right card for the old computer. Gettin a new computer, but I still want to upgrade the old one. If I bought a 1.5v 64-bit or 128-bit would it work? When I run the dxdiag it says that its an AGP GeForce FX 5200 128MB 32-bit 800x600 60MHz. With the AGP 1.5v 32-bit 66MHz Bus slot, could I upgrade it to a 1.5v 64-bit or 128-bit card? Was wanting to upgrade it to a 7600gt. I fugure there are agp cards out there better than the 5200. my 5200 is a 2002 or 2003 version.

Thanks for any advice I can get.