PCI Express vs AGP help

so… the misses and I bought Black and White 2 recently, and to say the least, at some points it runs worse than me on a treadmill.

I had a good look at a new GPU (mainly the GF7800GTX model, or the new ATI 520 (once it’s out)…

I noticed I can’t really get any new card that’s still AGP, except the ATIX850XT… and I don’t want a small increase in performance, I want a BIG :drool:

I never really got into this PCI Express deal… can anyone guide me to what I need? I heard I had to get a new Motherboard and processor too… (and when it comes to what board to get, I’m lost… :()