Thanks! I'll try that out. I feel like an idiot, but can I find out my motherboard/CPU etc. without having to open up my computer? I've upgraded some parts, (mainly graphics card and PSU three years ago) but the rest came with the computer. I customized it at HP's website, but mostly went with what was recommended when I ordered it.
And I'm looking at two cards: one requiring 500w minimum, the other 550w.