Do not buy the 9800GT. It consumes about 80w - 83w of power. More than is recommended for a 300w PSU; unless you are gonna buy a new PSU. It requires a 6-pin power connector. Since it is "open box" you don't know why it was returned.
The HD 5670 consumes under 30w so no additional 6-pin power connector is necessary. It is also new. But you need to deal with a mail-in rebate.
See following HD 5670 review. I think the 9800 GT is about a powerful as the GT 240 in the review. I could be wrong though. I do know it is less powerful than the GT 250.http://www.www.xbitlabs.com/articles/video/display/radeon-hd5670-hd5570-hd5450_2.html#sect0