If you're going with Dell or HP, keep an eye on
www.xpbargains.com. From time to time Dell will offer significant rebates on systems and I've found this site invaluable at finding these rebates. I have good friends who have purchased PC's and laptops from Dell at 20% off based on coupons/rebates from that site.
I'm generally in the build-my-own boat too, but there's nothing really wrong with any of the major brands anymore. Dell, HP, Toshiba, Sony - they're all about the same. However, my wife used to be in charge of the tech support for a 1,000+ student college and she always said that they had the fewest problems with Apple and HP machines out of all the others. YMMV.
Get something with an Intel Core2 Duo. Note the '2', and note the 'Duo'. At the risk of a flame war, that's probably the best processor on the market right now. Absolutely nothing wrong with AMD based systems (I have 2 at home), but Intel is ahead of AMD for the moment. As a database administrator and application developer by day, I can tell you that the Core2 Duo technology really works and is really great when doing multi-tasking operations. You'll be amazed at how your PC will be able to do autocad renderings and index your MP3 collection at the same time.
Get something with a 7200rpm hard drive. There is a difference between a 5400 and 7200.
Get something with at least 1GB of ram. Don't skimp on this. You just simply can't have too much RAM. 2GB is better if you're going the Vista route.
