I took advantage of that CoolerMaster case + PSU bundle which several sites are doing for about £42 at present (Elite 330 and 460W), and added an Asus P5KR mobo, an Intel E6750 processor (more on that in a mo), 2Gb Corsair RAM (800Mz flavour) a Samsung 500Gb drive, a Pioneer DVD RW and the Sapphire Radeon HD2400XT graphics card. (I love looking at the latest games, but never play them long enough to justify adding a nought to the cost of a g card.) Oh, and Vista Premium (OEM). Total cost, with postage, was about £450.
My only PC building experience until this past week was adding a bit of RAM to an old Dell and a wireless network card or two elsewhere. Then a couple of years ago I managed to completely b*gger up the fitting of a second hard drive in the main family PC which resulted in the death of the original drive. Not a great track record, BUT when I read about the E6750 I just had to have it and the only way to get one in a Dell (my first port of call over the years) was to buy one of their XPS gaming rigs - £600+.
The other huge advantage is that the new PC is much more upgradeable without those irritating Dell nuances and in two or three years' time I won't have to buy a whole new machine to get extra speed or capacity.
Scary but fun and definitely very satisfying. To anyone on the verge of going for it, do give it a go.