Though the X2600 Pro AGP would be the best deal for you right now, even an X1650 Pro would be a decent upgrade to a 9800 XT. See Tom's December 2007 GPU guide page: "What about this other card?":http://www.tomshardware.com/2007/12/03/best_graphics_card/page6.html
You can use this hierarchy to compare the pricing between two cards, to see which is a better deal and also to determine if an upgrade is worthwhile. I don't recommend upgrading your graphics card unless the replacement card is at least three tiers higher. Otherwise, the upgrade is somewhat parallel and you may not notice a worthwhile difference in performance.