Ugh... a hard choice... I bought a bargain basement Dell last October and was really dissapointed when I found out it only had PCI slots. Not even AGP!!! No wonder they were dirt cheap....
I thought people don't make PCI motherboards anymore.... anyways to cut a long story short, I ended up buying the best PCI graphics card I can get my hands on and my systems still sucks when I run the latest games.
If I were you, just bite the bullet and get a new PCI-Express capable system.
Then again, maybe you can just hold on a few months and buy your next PC from Apple. The forthcoming Intel PowerMacs have PCI-Express slots and will run both OS X and Windows