Depends.. Are you talking desktop, or laptop?
If you're talking a desktop, I'd say build it yourself, put the right hardware in it, and enjoy. A laptop is a bit harder. Buy one preinstalled.
I recently went through and searched the various vendors that sell linux-based laptops. I've had dells before that I've put linux on, and 1 HP. They all worked to an extent, but getting wireless to work on them was a pain. I couldn't game well on either one due to the video cards either. After a lot of searching, I found a company that offered the features I wanted, and a decent price. All told, it was worth the time, effort, and money to get the hardware I wanted.
is where I finally bought mine. Serval performance, and I couldn't be happier. They provide online support via the ubuntu forums, and between them and the community there I have had no problems.