Okay, my suggestions are build it yourself, you'll probably save 40%.
If you don't want to, here are some suggestions:
P4 CPU, needs to be greater then 2Ghz
Athlon XP, needs to be greater then 1.6Ghz.
DO NOT UNDER ANY CIRCUMSTANCES GET ANYTHING WITH PLAIN SDRAM!!! ESPECIALLY A P4. You will want DDR for a Athlon and RDRAM(Rambus) for a P4, DDR will also work with a P4, but it's slightly slower.
80GB atleast. You may not need that much(you always need more actually), but it's so fucking cheap that alot of vendors are screwing people over with old stock 40GB that they purchased months ago at higher prices. You'll also be better off with a 7200RPM drive, though 5400RPM will not kill you I guess
Really doesn't matter much, most stuff is pretty good, even onboard things.
Don't get anything that has "LE" or "MX" or "PRO" in the name. Also avoid anything with the word "integrated" or "onboard". You'll want either a Radeon 8500, or a Geforce4 ti4200. Both are cheap enough that they shouldn't be out of your range.
Faster the better... Should be able to get a 16X for cheap.
whatever it comes with :p
17" BARE MINIMUM, 19" or large is very nice. I can have 8 full expanded video tracks open with the Premiere video preview full expanded @ 720x480, it's very nice.