If ur gonna spend $300 on a card, DO NOT get a 9800, its a waste...its outdated, whats the point just stick with the card you have b/c the quality gain isnt gonna be that drastic, and for sure not worth the money. Either jump into the next series of cards (6600GT or 6800 or x800) or keep ur money till u upgrade your mobo and do it all at once.
The 5700 is still a good card, if i were u i wouldnt upgrade now.