I recently won a complete G4 Gigabit sans case on eBay for $90 shipped. I got the Logicboard, 400 MHz CPU, 512MB Dimm, AGP Rage 128, Power supply, and DVD-ROM.
I bought a used Antec ATX from work for $8 and about $5 for some other misc stuff.
So here it is a Beige G4.
I added 3 256 MB DIMMS that I had in my G3 for a total of 1.2GB RAM. I also added an ATTO U2LD Ultra80 SCSI card, a 18 GB 10K RPM Ultra 160 Hard drive, and 30 GB Quantum HD from my old TiVo.
It took me about 8 hours to put together. I had to drill holes for the stand offs as none of the ATX ones lined up. There is a chip on the backside of the board that requires a heat-sink. I had to grind down an old 440BX north bridge heat-sink until it fit. I cut the I/O plate from a piece of roof flashing that I bought at Home Depot. It did not come out as well as I had hoped. I miss-drilled the holes for the audio ports so I ended up just cutting out a big hole for the both of them. Cable management took a bit of work but I am happy with the results. I just need to go buy some razor blades so I remove the stickers for the old motherboard.
In the near future I plan on upgrading the video card and possibly a Sonnet 1.0 GHz CPU. My old G3 was a 450 with a PCI Radeon and this one is so much quicker. So I am having a hard time convincing my self to buy the CPU upgrade.