well since you're just looking to upgrade in the $250 range stick w/ agp.
you can get a refurb x800xt platinum for $229 here
http://shop.ati.com/searchresults.asp?dept_id=39
it comes w/ a 3 year warranty straight from ATi.
here's the lowdown on PCI-e
-up until the 7800GTX there was no noticeable difference in performance between them
-PCI-e has more potential and is a better all around design than AGP, but if you already have an agp board there's no reason to jump to PCI-e now.
-the last AGP card from nVIDIA was the 6800 series. they will not be making another.
-the last AGP card from ATi will be the upcoming R520 "fudo". how much faster the PCI-e variant of it will be is yet to be seen.
-PCI-e cards are becoming cheaper than AGP cards steadily as AGP is phased out (minus the budget $100 cards).
-almost all new chipsets will be built around PCI-e
-current best chipset for gaming is nForce4 ULTRA. BFG makes a good version w/ all the goodies & lifetime warranty available through newegg.