I just bought a HP a6400t desktop pc from best buy. I'm unable to play current games, for example call of duty 4. I found out that I need to install a video card. I was looking at the Nvidia 8600 GT 1 GB. According to the specs I need a 350W power supply. This pc has a 250W power supply. I would like to upgrade the power supply, but before I purchase, I want to make sure it fits in the case. Are there standard sizes?