What are you planning to do with the box?

Last year's mid-range is a decent option. Particularly if it comes from a dumpster, friend, firesale, or other choice. Note that general used venues (thrift stores, general-interest flea markets) are generally filled with grossly overvalued boat anchors. Used electronic supply can be a real bargain, as is your local computer user group.

Other than that, I'd aim for a high-end monitor (CRT still beats LCD, though it's getting close), solid case and mobo, more memory, ports (USB, Firewire, GigE), and a fast disk as higher priorities. CPU, internal modem, keyboard/mouse, and chrome are far lower. Video really depends on what you're doing, but unless there's a lot of high-end gaming, $40 is going to get you a card that will handle anything you throw at it.

I'm with Scott on the $400 box. Selective cannibalization of existing HW may also serve you well.

Curves: watch for incompatible memory and peripherals: AGP video is current, PCMCIA 16 v. 32 bit. USB1 vs. USB2. CPU socket types. Pin down Greg or the other gearheads for other specifics.