Just added an XP2900 to the farm. I built the box today so I won't look in to OC capabilities until it's been properly burned in for a few days and even then if it'll OC I'm usually somewhat conservative about it.
The source? I got a free Compaq Presario 5900Z from a coworker. An old Athlon Classic 750. Didn't work & he'd stripped the HD. Looked like the mobo was dead as there weren't even any beep codes when powering up. So I basically gutted it. Found a deal for the Athlon XP2900 + Abit K7V for $100. Added a new PSU as the Compaq was 145 watts and lacked the extra 12V connector, 512MB RAM, Radeon 7000 (no fan; minimum card with SVGA + DVI), HSF, & case fan. Moved a Samsung 160GB HD from my main PC & installed a vanilla XP Pro SP2. Basically, what remains of the 'Compaq' is the case, DVD-ROM, and LiteOn 48x CD-RW. It's amazing: XP Pro + AVG + Firefox + the Folding client + various mobo & video drivers = 3.7GB. Still, not a bad PC so far. Decent speed.
There were other options, of course, but I wanted to keep it fairly cheap. The only cheaper mobo/CPU solution was a Duron 1.8 combo for about $30 less. Since I knew it'd be Folding, I opted to spend a few more for the Athlon XP.
This machine will hopefully be the start of my media server. I've a Maxtor 200GB drive to move in to the chassis shortly. When that runs out I'll get serious about storage. Hopefully by then the 500GB disks will be shipping & will start to be affordable.