Depends on what you want to play in the future. When you said Battlefield did you mean Battlefield 3? If not and you only play non demanding games like CSS you could get something really low, GTX560Ti would be the lowest I personally would go but I could see you getting something even cheaper given anything will be faster than the 9800GTX.
*Insert negative stuff here.*
"..hiding your [story] beneath endless layers of confusion doesn’t make you an artist, it just makes people think you are."- Gameshadow