I was looking at upgrading my box about 3 months ago, when I saw this:
Dell 400SC on XPBargains.com
Every few weeks they'll have a $100 or $150 rebate on a fully-unloaded Dell PowerEdge 400SC. It comes with the same motherboard (Intel 875 chipset) as the 8300, it has serial ATA, 4 total slots dual channel memory, P4 (I upgraded to a P4 2.8 w/hyper-threading), 6x rear USB 2.0 (front slots are covered up by Dell's PowerEdge server case). And did I mention the case is quiet? No system I've ever built has been quiet.
I only took their 1x128 MB ECC PC3200, and ordered 2x256 and 1x128 from crucial.com to make 768 MB. All told, I think $600 got me this: P4 2.8c on an 875 chipset, 768 MB RAM, 40 GB ATA/133 hard drive, floppy, and 32x CD-ROM. It's a great start to add your own hard drives, firewire card, etc. The kicker here, of course, is the 2.8c. They're pretty pricy, but it's only a small premium over the Athlon32 platform to get the HT performance that really shows up in digital video editing.