Because the 4870 is one of the most power hungry GPU's on the market, and Rosewill PSU's are just about the worst PSU's on the market. A 500w PSU from a reputable company would be on the limit of acceptable, a 500w unit from a fifth tier company is not even close. I would never use a rosewill PSU if it could be helped, but if I had to I would only trust it to run integrated graphics at the most, no overclocking of the CPU. Rosewill PSU's blow up, and having a powerhungry GPU sucking power from it just makes that more likely. The PSU is one of the most important components in a system, you should never go cheap on a PSU.