If there's a Frys Electronics near you, I highly recommend the Build Your Own route.
They're currenlty running a special again, AMD Athlon 1.0G BYO system for $230.
It's a case with the motherboard already installed, along with a NIC and a modem. The board has onboard sound and video. It comes with the CPU chip which just needs to be plugged in, and have its fan mounted on top (takes all of two minutes).
Buy some memory, and cd-rom drive, and a hard drive, and pop those in and connect the cables. It's very simple.
Figure
$230 for the main components (case, board, modem, nic)
$120 for a 30G 7200 RPM drive
$30 for a 40x CD-ROM
$50 for a 256M stick of RAM $430 for a nice system. Can't beat that. If you've never assembled one before, it might take you a couple of hours but its nothing difficult, especially since the motherboard is already installed.
My main computer took me about 2 hours since I had never done it before. My Unix box (which is basically what's listed above) took me less than an hour.
--mark