If you don't generally build your own computers, I'd really recommend going with one of the decent packaged systems. If you're not a power user or a heavy gamer, Dell has some very nice systems that you can add a few things making a very good system for the value. A couple things you want to be sure to get are:

RAM - AT LEAST 1GB (gigabyte). RAM is the single best performance to dollar upgrade.
Large hard drive.. at least 80gb. You'll be amazed how quickly it fills up.
Good video card. I personally like nVidia cards like the GeForce 6800 but some of the ATI like the 9800 Pro or even the All-In-Wonder cards if you want to do any video editing.