with that PSU, assuming it puts out a true 280w (most likely does not) you have little to no options. There are a few GPUs you could use, none that come to mind. Honestly you'd be better off saving $500 and building a new system - at least upgrades from that point on would be easier if you buy yourself a solid case and PSU
G31/33 chipset is very slower, so if u upgrade gpu the mobo is not fully utilize the gpu speed. so don't waste money. upgrade mobo to 41express chipset Intel mobo (ddr 3) this mobo support u r processor and integrated 1gb Intel gpu Is enough for u now. And no need to upgrade psu.