PCI-e is just the new expansion slot type all the new vid cards are using. Better through put then with pci/agp for the higher end cards.
ATI makes the card. Pro and xt are just different models of the same chipset. XT is a little bit faster. Looks like newegg only has one XT in stock and it's only 256mb. 512mb pro would be about the same, maybe slightly better.
http://www.newegg.com/Product/Produc...d&Order=RATING