Cheaper to buy a complete machine than build one. If you want a multi-year warranty go to someone like Dell, Mesh etc.
Unless you are an extreme gamer, any machine with a CPU faster than about 1.6Gb is fine. 512Mb RAM is OK, 1Gb is all you need. You want a PCI graphics card, sweet point for price/performance at the moment is the NVidia 6600 or ATI X700 range. Get DVD burner, they only cost around 25 pounds these days.