I've been building my own PCs to use at work for the last year or so. Allows me to upgrade when needed and keeps the crappy $399 Dells away from my office. Currently(April '07) I have a classic Antec/Chieftec case with an Abit AW8-MAX and a Presler Pentium EE/XE965. Complete overkill for running Quickbooks, Outlook, and a few specialty apps, but I had the parts laying around so why not? Hey, I don't pay the power bill, and during the winter the extra heat was great. Due to the layout of the office, the system sits pretty much at my feet and all the extra heat flows out from under the desk right onto me. Now that it's warming back up, I'm going to be replacing that box with a custom built small form factor. They make me wear pants, so I really don't have much choice if I don't want to melt this summer.
I've got worklogs at a few websites, but rather than rehash, I'll just throw out the specs and get to the pictures.
Intel Core 2 Duo T7200, 2GHz, 4MB cache
1GB G-Skill DDR2-667
Onboard Intel GMA950 video, good enough for Vista Aero
DVI, VGA, and Component video out
Onboard Realtek 5.1 sound
Onboard Intel Gigabit Ethernet
Atheros 802.11b/g wireless with external R-SMA jack
Seagate 7200.10 500GB SATA drive
Panasonic 8x DVD+/-RW Slot load slimline drive
generic zillion-n-1 flash card reader
100 watt power brick, board has onboard power circuit with 8-24 volt input
All cooled by a single Yate Loon 120mm fan
 I may swap out the 500 GB for a smaller drive. I can use the 500GB in my next project.
And the pics:
Finished size is 9" tall, 6.75" wide, and about 7" deep. Makes almost no noise, only noises are when accessing the optical drive or the hard drive. If I do change the HD, it'll be for a Seagate that I know is extremely quiet. Power usage at idle is about 30 watts, full load of around 50 watts. That's about 20% of what the XE965 system uses, and the overall performance of the two is quite similar.
I need to get some software installed on this before I take it to work as well as make sure I can get the custom printer I use working. Once those are done I plan on finally ditching the space heater.