I'm not sure why you would ever want to buy a "trading PC". Surely all you need is a good processor, a broadband connection and as many monitors as you can lay your hands on? For me if I was upgrading tomorrow I would get a medium spec dell desktop (£600), buy a G400 matrox video card (£200)and then get 3 17" TFT monitors (3 x £300) for a total of £1700. That would definitely be an excellent start for a trading PC.
Anyone care to differ? ;-)