In some ways, putting together a computer is a lot like putting together a home theater. I'm more knowledgeable about the former than the latter, so here are my recs (with the appropriate HT analogy
)
Processor: Think watts per channel. It doesn't hurt to buy as much as you can afford, though even the entry-level will probably be adequate for most of what you're doing. I don't know about the requirements for AutoCAD, but otherwise, even the cheapest processor could meet your needs. Like WPC, comparing clock speed (GHz) within the same chipmaker's line gives a rough estimate of performance, but there are diminishing returns as you get higher and comparing clock speeds between chipmakers is somewhat meaningless.
RAM: Think amplifier. You can never have too much. While you can still hear music/run programs at the low end, having more can make the experience more pleasant. Of course, there will be diminishing returns for higher prices. If you only upgrade every few years, then get a computer with at least 1 GB of RAM. I would get a slower processor with 1 GB over a faster processor with 512 MB.
Hard Drive: Again, think amplifier: it never hurts to have too much. When comparing different hard drives, a rough rule of thumb is $0.25/GB is a good deal, and $0.50/GB isn't.
Video Card: think video processing on AVR's. What comes included will satisfy most people. If you can't think of why you need more than that, then you probably don't need it anyways. However, check to see if AutoCAD needs specific drivers or video cards.
Windows Vista: Think HDMI. It's the wave of the future, it's supposed to make your life easier, but it's still somewhat unrefined currently. There's a decent chance you don't need it now, and can wait until the next upgrade (even if that's a couple of years away) before getting it. I wouldn't pay extra for Vista, as there are some incompatabilities with some older programs.
