My main PC isn't a desktop at all, it's a Toshiba Satellite 1905 notebook:
http://www.pulsemed.org/toshiba-satellite-1905.htm
Its power supply is max rated @ only 15Watts.
Rather aged @ about 2 years it's the fastest PC I have and does all my heavy Lightwave 3D proccessing in quick order.
P4 2.4Ghz
512 RAM
Sharp, clear 15" TFT active screen
40GB HD, built in 3.5", buit in wireless, DVD ROM, CD RW, Firewire, etc
I picked it up @ Bestbuy new, lucky me due to a return quirk was about $800.
(About $1,000 off shelf price at the time)
Someday I'd like to get back to a desktop but am waiting for the LCD based monitors to get higher resolution. I hate working with a ~2sq ft CRT based monitor taking up desk space.
My old PC is a pig. Compaq Celeron 600Mhz 256K Ram, and a 21" monitor that practically makes the lights dim for a moment when you turn it on! The kids us it now.