I just specced out a full sized desktop comparable to the imac 2.8ghz core 2 extreme, and it was about $1400. Plus labor, plus additional clutter and desk real estate. No warranty. No operating system. No keyboard, no mouse.
So add $175 for vista ultimate oem, and you're at $1575. Add $50 for a decent keyboard and mouse . $1625. So you're saving about $600, which is a decent amount of savings. But you're not getting the warranty, you're not getting the integration, and you're not getting the form factor. Not to mention, you're still not getting OS X, although theoretically you could get the hacked version that would run on that system.
But really, the high end is just listed on the website to show it's possible. You can get a similar configuration but with the 2.4ghz processor and 1gb of ram, and a 250gb disk refurbed for $1450. Downgrade the above home built system to those specs and you're at about $1300.