Get a DDR based mobo, it will make a difference when it comes to encoding videos.
Also 512 Mb will make a difference, since you gonna be working with large files.
Get a fast hdd, WD SE with 8mb cache, IMO.
About the chipset of the mobo, here's what you get in my opinion.
VIA KT chipsets are faster, but you might find some incompatibilities, and normally depending on the brand of the mobo it's more expensive.
SiS chipsets no big issues, little slower than the KTs, but normally cheaper mobos.
Personally I would stick with Epox 8K5A2+ (VIA KT333) mobo, nice features, has RAID controllers if decide to go with RAID.
And probably you can OC ur XP 1600+ to the limit with some nice PC2700 RAM on it.
Good luck on ur pick!
Raf