I noticed that when I went from a 64mb to a 128mb PCI card, that I could set games to a higher detail setting. For example, when I played Max Payne with a 64mb card and set the details to high at 800x600x32, the game would crash back to the desktop. I could only play it at medium settings at 800x600x32. Whenever I switched to a 128mb card, I could suddenly play the same game at high detail settings at 1024x768x32 with no drop in framerates. You see, PCI cards cannot do texture swapping like AGP cards can. So, many times if you fill up the memory buffer on a PCI card, your performance will either drop to unplayable levels (sometimes in the 1-10fps territory) or your game may crash altogether. When looking at a PCI card, you want to get as much on-card memory as you can afford.
EDIT: $49/after rebates is hard to beat for the 64mb 9100 - that's the best price I have seen so far for one of these cards.