Where do you intend to buy the card. The GTS 250,4770, and 4850 are usually around the same price as the 9800GT. This is the order of their performance 9800GT<GTS250<HD4770<HD4850.
THe only question would be whether your power supply is enough to hand the cards. Could you post your system specs, specifically your PSU. Also, what sites are you looking to buy from?