i want to uprade from a 300watt psu. i need to know if i need to worry about... 1. size, or are they all about the same size? 2. types of pins and what-nots, or will any psu i buy work with any system? 3. what brands are good and how much should i spend? i have a compaq presario sr1630nx with 512 ram and a xfx geforce 7600gt.