Building a Ultimate Media Machine
I'm regularly asked what's the best computer for [insert media project here]. My typical answer is the best computer for your project is the best one you can afford; with the caveat you should meet the recommended (not minimum) system requirements for your project. Jason Dunn recently put together what I'd call a great computer for almost any media project, combining a compact Shuttle SD39P2 with an Intel Core 2 Duo Extreme X6800 and other handpicked components. If you plan on using a new PC for editing HD, my only change to Jason's configuration would be a video card (preferably ATI if you install Vista) with 512MB of dedicated video RAM; smooth playback on the video editing timeline is easier to achieve by maximizing graphics resources.
For a complete rundown of components used in Jason's Monster Media Machine, including performance benchmarks read Building a Monster Media Machine at Digital Media Thoughts.