I am a novice when it comes to modifying my computer...I have a HP Pavilion a1357c Desktop and it has a 300 watt PSU. I wanted to upgrade the video card to something like an AMD Radeon HD 7770 or around that performance level, but I believe I will need to upgrade the PSU. Can anyone tell me a good PSU between 400-600 watts that are good for this model? Or how can I tell on my own? Thanks a lot for any help.