First off, why can't you just hit up Anandtech's front page for its X700XT and 6600GT reviews? Or hit up Google or Neoseeker?
Anyway, both are about 15% faster than a 9800P on average, and probably somewhat less so when AA+AF are involved. And the X700XT won't hit AGP for a while. So an upgrade is really not smart from a price/performance perspective. (Say you sell your 9800P for $160 and buy a new 6600GT for $200. That's +30% for a card that's not that much faster.) But you may have other reasons for upgrading (Linux drivers, Far Cry HDR, better OGL performance), in which case $40 isn't that much to pay.
As a general rule, bang/buck always goes down dramatically as you move up the price range. You're basically buying higher resolutions and levels of AA with the pricier cards, but a 6600GT (and even a 9800P) will be pretty darn playable at 10x7 with some AA.
Check out Anandtech's, Hardware.fr's, Hexus.net's, HardOCP's, Xbitlabs', digit-life's, etc.'s X700XT reviews for direct comparisons b/w all three cards.