Rather than building one from scratch, you could buy a box that's certified to run Linux. Unlike the old days, I find that nowadays you really can't build a box any cheaper than you can buy one from companies like Lenovo or HP, and Lenovo has several boxen that are "Linux ready."
Personally I think my box-building days are over. I no longer play video games, so all I'm really interested in is a fast CPU and a PCI16 slot for my "silent" (no fans) video card, and audio and networking that are supported by Linux (which is pretty much everything Lenovo sells; I haven't looked into HP -- I don't like their reliability ratings.)
Regardless, I couldn't come up with pricing any better than about $800 + tax + shipping no matter how I scrounged, and that was only for a Core i5, not a Core i7. It's about another $150 to bump up to an i7, but I don't *need* a quad i7 for what I do. I'd *like* one, but I don't *need* it.
I think I'd get more of a performance boost out of using my PCI video card to offload the memory access from the CPU channels than I would out of bumping from an i5 to an i7.