i have to agree with everyone else, and go with build. buying a pre-built you're paying for labor and software that is of no use to you.
However, if you do want to look at pre-builts, check out Acer. they are real good quality and are the only company i know of that 99% of its tech support is based in America (texas to be exact).
for parts, if your in America go with newegg.com. They do not delive outside the US so if you don't live in the US it's kinda pointless to look there. I'm not sure if Tiger Direct ships outside the US....