I have been running BOINC on four Ubuntu Linux (free download) systems for over a year and have had no problems whatever. Also run BOINC on four Windows systems with no problems, but one OEM copy of WinXP Pro costs over $100. Another consideration - Ubuntu Linux has numerous updates which must be manually downloaded and installed on each machine. Can be a weekly chore with numerous machines. However, if Ubuntu updates are ignored, BOINC performance isn't affected in my experience. Of course, Windows has a good automated update system requiring almost no attention. I can't factually address the relative speed between Linux and Windows, as I have no data nor statistics. All of my systems have different CPU's, RAM, and Mobo's.

I've built several machines solely for the purpose of running BOINC WCG and here's my harware "requirements".

CPU: Intel Q6600. Best bang for the $$$ CPU there is and has been seen selling for as little as $200. It can crunch 4 work units at once and can be overclocked nicely if you want to do that.

RAM: 256MB per core is enough, ie-1GB on a quad core. A name brand 1GB stick of PC2-5300 can be bought for under $20 these days. Just use a small OS such as Linux or Windows Server 2003 and you'll easily have 850MB left over for BOINC to use.

Motherboard: Don't go too cheap here! My personal favorite is the Gigabyte P35 DS3L. It's a solid board and overclocks nicely. Just beware of the CPU voltage droop and adding a few RAMsinks to the mosfets helps keep them cool.

cooling: stock cooling for stock speed, high end air or water cooling for overclocking.

power supply: No need for anything fancy here but don't cheap out either. I use the Sparkle Power 400w PSU, it can be bought for around $35. Rock solid and reliable.

graphics: there's no need for a graphics card. Most motherboards will boot without one. I keep an ancient PCI video card around for setting up new computers and for the occasional time I need to access the BIOS. Outside of that everything else can be done via BOINCview or Remote Desktop.

Operating system: free is always best so Linux is probably your best option. I really don't know if Windows or Linux is better for crunching but I doubt there is a large difference. Personally I use Windows Server 2003 x64.

Hard drive: anything will do, even an ancient 10GB drive. Often you can buy large lots of outdated drives on eBay for a few dollars. The only downside is older drives tend to be noisy.

networking: buy a cheap network switch. Plug all your crunchers into that and then plug the switch into your router.