I got a pc recently from cyberpower and am pretty happy with it. I was able to customize all the parts and the price was competitive. I have used customer service once so far and the experience was good.
I purchased a trading computer about a year ago from Trading Computers and I am super happy with the performance and the exceptional support. Then again i use to use Dell and its like switching from a Nissan to a Ferrari. Another place i hear that good is "i buy power" but never tried them.
I went for broke and bought a top-of-the-line Alienware Area 51. I opted for liquid cooling, i7 960, 64-bit OS, and dual SLi with nVidia GTX 295's (which I run in non-SLi for three monitors). Yes - a lot of money and probably not neccesary for trading, but I haven't looked back. I purchased the beast this past July.
I liked it so well I ended up placing an order for a similar laptop this past January. It weighs a little under 20 lbs and drains the battery in about an hour. I use the computers for things other than trading (I run some processor intensive simulations and play the occasional game) - but the laptop was overkill and is a beast to carry around.