Well, to add an update, i ordered a 650W Antec PSU from newegg.
Newegg.com - Antec NeoPower 650 Blue 650W ATX12V / EPS12V Power Supply 100 - 240 V UL, CUL, CE, CB, FCC Class B, TUV, CCC, C-tick - Power Supplies

After putting it in today, the computer ran fine. Then i put in the HD 4850, and plugged in the 6-pin connector, and hit the power button. The blue light on the PSU lit up for a moment, then went out, the computer wouldn't power up. I'm beginning to think that this is a problem with the GPU instead of the PSU