I bought my trading computer from www.tradingcomputers.com It is the best computer I have ever used for trading. Good luck.
"Building" a machine isn't quantum physics, its easier than what you or your kids have done with legos. I can't believe anyone that is serious wouldn't take the few hours it takes to learn to build and trouble shoot the fundamental tool of their business. For half the price of those "trading computers" you can order 2 of each part, build your own machine, image the harddrive to the duplicate and if something goes horribly wrong during market hours the most you will ever be out is an hour as you swap out parts and find the problem, then order another duplicate of the parts you replaced. Sorry but anyone who pretends that "building" is such a pain probly has an alternative motive. I mean if you really want to be sneaky just ask those places what parts are in the machines and order them yourself.
This may give you some idea what to go for in way of CPU and MEM.. http://www.sharkyextreme.com/guides/EGBG/article.php/10721_3771096__2 Sharky builds a new computer every couple months
i would suggest just building your own computer too. it took me more time just trying to figuring out what parts i wanted for the computer then building it. this was my first build also. simple as cake. specs intel quad q6600 2.4 xfx 680i lt sli 2gb ocz sli ram 6400 corsair tx650 thermal ultra extreme cpu cooler 2 8600gt xxx antec 900 case if you don't want to build one. here is a link to one i just found that is similiar to my specs. http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=3528119&CatId=1815 and around the same price when i built mine. u'll need an extra video card though.