i think PCI Express makes all the difference. i don't know what sort of gains it has over AGP if everything else is controlled for, but from what I've read, in general, PCI Express seems to be a major improvement over AGP.
Does this game run better with one than the other (ATI or Nvidia)? Which one did the developers prefer? Also, CPU-wise, is there much difference between Pentium and AMD? For this game in particular is one preferred over another?