I would go with a GTS 450 on the nVidia side or a 6770 on the AMD/ATI side would be your best choices. Your PSU might handle both cards since it is a dell. But for sure would need to know the what size it is in watts.
Then I would need to know how many amps are on the 12v rail. It is on a sticker on the PSU inside your case. The numbes look like this 3.3v 5v 12v 13a 14a. I need the 12v one with the 16a or ? Good luck.
Dont get an 8300gs, you will be disappointed the first time you try to play a game. Go with the previous posts and get a 6670, or 440gt. Oh, and its the power supply, not power chord some people need to upgrade. You actually do too in order to get a really decent card