What are some good computer part stores that carry a wide range of power supplies? I'm considering this mini-desktop from HP. It comes with a 230W power supply. I want to get a 300W PS for it because I'll be adding a dedicated video card. I found a couple on newegg.com, but how can I be 100% sure that they'll fit? For example, this one: http://www.newegg.com/Product/Product.aspx?Item=N82E16817104044 I believe what I need is a microATX power supply, but I don't see the actual dimensions there. Are they all standardized size?