My "rig" is essentially my HP dv5000t notebook. I'm a college student, it works out pretty well.
As for desktops, I have two P4 folding rigs in my dorm room (flat rate tuition = I don't care about a power bill).
Rig 1:
Pentium 4B 3.06 Northwood @ 3.2~Ghz (stock volts)
Masscool socket 478 HSF (works pretty well)
Gigabyte IPE1000 (865 chipset)
1GB Kingston DDR-266 ECC
$20 Xabre something-or-other video card
420w Thermaltake Purepower
80GB WD IDE hard drive (model is irrelevant in this case)
Old manilla white case I had laying around
Ancient generic 48x CD-ROM
LG DVD-ROM
Ancient Yamaha PCI sound card (merely for offloading sound from the CPU)
Rig 2 (old "broken" Gateway I got for free):
Pentium 4C 2.6Ghz Northwood
Stock Gateway HSF
Gateway motherboard (865 chipset)
Integrated video at the moment
512MB generic DDR-333
Cheapest generic 320w PSU I could find on Newegg
20GB WD IDE hard drive (ooooold)
I have a 3500+ rig at home I gave to my little sister to use. Better than both of those...by far...I'm nice like that.