Linux has now matured enough that there are now system integrators who will assemble a workstation for you, install and configure a Linux, and do an intensive burn-in to test it before it's shipped to you. If you have more money than time, or you have stringent reliability or performance requirements, these integrators provide a valuable service by making sure you won't get hardware that's flaky or dies two days out of the box.
There are several firms of this kind (and I'll list them here as I learn more about them). The only such outfit I know about personally is VA Linux Systems; in 1998 I became a member of VA's Board of Directors. VA builds high-end, high quality Linux workstations with a nifty Tux-the-penguin logo on the front. They have intimate ties to the Linux community; SourceForge, the GNU site, and the Debian project all live on a machine in their back room, and Linus's personal machine is one of their boxes.
For those of us without a champagne budget, the rest of this HOWTO is about how to install Linux yourself.