from my understanding the move from the 8000 to 9000 series was largely due to a die shrink and little else. so the 9800 will be slightly better but not by a large margin, it'll prolly overclock better then the 8800gt too.
yeah the 9800gt will be a hair faster then the 8800gt. From a quick glance at prices loot like it's a 50 to 80 dollar difference between the cards. IMO I don't think it would be worth it, espeically with the radeon 4850 dominating the 200 dollar price point.
If you own neither of those cards right now, go for the new ati ones instead. Faster and using slightly less power (means less heat). The 4870 has dualslot cooling like the nvidia ones btw - quite handy in a closed enviroment.
I already have the 8800 GT...was noticing the price going down so I was trying to see if it was worth trying to go after the 9800gt. Unfortunately, my brother already cut out the UPC on the box for a rebate, so I can't exchange anyways. The 8800GT works fine. Maybe next year I'll upgrade.