Now I want to build a rig with SATA/SATAII and PCI Express. Just go all out and see how much of a diff it makes.
The difference will be in the parts, not the interface. PCI Express has some big advantages over AGP, but not to "normal" gamers. If you're using the video card for rendering and putting the rendered image back to disk (machinima or SFX) then it makes a huge difference. It can also support multiple video cards which AGP couldn't (thus all the dual video solutions coming back).
SATA really makes no difference... but a lot of the new SATA drives have NCQ (Native Command Queueing), which can be a big help, plus higher bit density leading to higher transfer rates, etc.
Oh, and the narrow cable for SATA is a bonus to case cooling.