Just buy the parts from like Tigerdirect or NewEgg and then look around where you live and there's bound to be a small PC shop that'll be more then happy to set it all up for you for a small fee if you're not comfortable doing it.

You will end up saving so much more money custom building your own verse buying a prebuild PC from some company. There are some forums (Tomshardware for example) that you can post on saying what kind of games you're going to want to play and there's some folks that'll put together a PC which takes the guesswork out of it and still saves you the money verse buying a prebuilt PC.