DEF. BUILD! If you dont have the knowhow, get a friend to if you can. Built mine from scratch and saved a bundle, but there are + and - to the situation.
By building it yourself you give up that wonderful (but in most hellacious) tech support.
Although by building you can save hundreds of dollars. For example, since im in college, I was able to purchase Windows XP Pro. for $10...yes $10 (site license)! Even so, just a few weeks ago i built (theoretically for a friend for a price) a top of the line computer with everything you could need, including the new AMD X2 3800+ (I think), and good graphx card, mem, etc. It rounded out to be around $1400-1600 (maybe less, cant remember exact figure). All parts were chosen from Tigerdirect.com and being a NC native all i would have to do is pick up the parts from the store. This computer was easily compared to the top-of-the-line Dell XPS system, or some Alienware systems.
Sidenote: Alienware is a bad comparison, yes they are expensive and over priced computers but they are awesome systems. The tech support is phenominal and lifelong as well, the bundled software is programs that you actually want. Many times also these systems will contain hardware and software that hasnt even been publicly released for individual purchase yet. Personally I wouldnt own one, but dont get me wrong, they are one bad ass system.