Suppose it really depends what you're asking when you mean barebones. What it sounds like, is that you just want a cheap system. But what it actually means is that you buy the parts separate and put it together yourself. Basically what my machine is. It's also a good chunk of the reason why they are cheaper.
You can make the same machine at retail from Dell or HP or someone, but without their branding and all the crappy software they pile onto it and stuff, and for cheaper. But cheaper doesn't have to mean cheap. You could run into the 10s of thousands if you really had that much money to throw about. =P
So look around at places that sell just parts that you can put together. Remember that you need to include everything, other than, of course, your monitor and keyboard and that, which you already have. But it's amazing how many people can forget something as little as the SATA cable to actually hook your hard drive up to the motherboard. =P Or a CD/DVD drive to be able to install programmes and drivers. =P
Make sure what you choose is compatible with all the other stuff too. There's no point getting a CPU that won't fit the motherboard, or a 200watt PSU for a system that needs 400.
If you are using a firewire interface, make sure your motherboard has a Texas Instruments firewire port, or enough PCI slots to add one. If you're over in America, I'm not sure what websites could help you out, but over here in England, Scan.co.uk is a great place.
3.8GHz i5 760/8GB RAM/Samsung 1TB Spinpoint F3s.
Shine WEB45, Vintage Metal Axxe Wraith, Stagg, Aria Pro II JS, Yamaha PSR-275, EKO Acoustic.
Studio Projects B1.
Studio One, Reaper.
Metal, Jazz, Ambient, Trance.