I've got a very similar setup that I use for audio editing, so maybe I can help.
That looks to be a very good setup, but I would make a few suggestions:
First of all, I don't want to knock the Athlon and get people riled up here, but the Athlon currently doesn't support SSE, and won't until the Palomino-based chips are out. I know the Athlon is faster clock-for-clock, but the one area where the P3 and P4 really shine is in multimedia editing and content creation. Plus, if you go with a P3 or P4, you can get a mobo based on one of Intel's chipsets, which in my experience tend to be more stable than VIA's, and make no mistake, stability is a HUGE factor when it comes to audio and video work. So if you're looking to buy now, you might consider a P3 with an i815 chipset-based motherboard, or even a P4, which supports not only SSE but SSE2.
Second, you may want to consider going SCSI and getting some 10,000 RPM hard drives. It's not absolutely necessary these days, especially if you're using 7200 RPM IDE drives - I use IDE drives myself, in fact - but you're doing video, not just audio, so you may want to consider it. If you decide to stick with IDE drives, I would recommend using IBM Deskstar 60GXP drives - they're the fastest on the market, especially designed for audio and video storage, and they're dead quiet, to boot. Unfortunately, they only go up to 60GB, so you'd have to knock a little space off your storage drive, but I think it'd be worth it.
Aside from that, the system looks a-ok. Let us know what you decide and how it goes!
