Ways to build up a farm:
1) Watch for fry's electronics deals, they usually have ECS K7S5A pro mobo and chip combos for a good price. I haven't been able to beat these.
2) Online buy/sell forums. You need cheap PCI video cards, power supplies, HDs, networking gear, etc.
3) Watch for me selling stuff. As jbcool mentioned, I sometimes get good deals on things like HDs (although these are mostly over). Right now, I've got 2 20GB WD HDs for $30 each, shipped. I also keep most farm equipment for 6-12 months, so usually twice a year I sell stuff. I might have some systems for sale in the next 2 months, for instance.
When building a farm, there are some very important factors.
The big three are power, space, and heat.
You need enough electricity to power the machines, enough space to keep the machines, and a way to remove the heat the systems generate.
One way to reduce electricity is with netboot, since you can remove the HDs.
For space, mounting the mobos on a shelving unit can reduce space. Heat is a problem, however. Removing the HDs can help, but you just have to have a way to pull the generated heat out. I dissipate heat into an 800 square foot room in my basement, and from there it exchanges with the concrete walls of the basement, as well as is assisted by the AC unit. However, it's hot and loud.
I hope this helps.