Should I use an "ATI Radeon x800 xt" AGP running on a 4x slot, or an ATI Radeon x800 Pro PCI-E?

Thanks, and the AGP one is 1.5 V non PRO, correct? Also, I am sorry for all of these similar questions, but which would run Oblivion better (the answer will make my decision about buying a new computer)? What kind of framerate would I get?