You will need to give the amps on the 12v rails. Watts dont always mean its what it seems to be.
I take it you have an AGP slot for the video card so you will be limited on how much further up you can go. I believe an ATI 3870 may be the best bet if I remember correctly.
Its also possible you will have to upgrade your power supply so get a corsair 450 watt or greater if you find Im correct.
Post back with the 12v amps and name brand of psu if you can find it...
AGP only goes up to a 3850.
I would recommend a 2600XT or a 3650. A 3850 may be out of your PSU's power envelope. For that matter so may the above two cards. Really depends on how they broke down the rails.
Ok first, what kind of slot do you have? Is it AGP or PCIe 16x. If its AGP you have nothing from nvidia that is available that is better then your current card. If it is PCIe then we still need the amps available on your 12v rail for your max GFX card. So, is your video card slot AGP or PCIe 16x, what does it say the maximun AMPs on your 12v rail is?