I think the best upgrade, as far as ENCODING/ Editing videos goes would be a new hard drive (preferably an 8mb cache/ 7200rpm one like the new Maxtor Diamondmax Plus 9 60gb at googlegear.com... http://www.googlegear.com/jsp/ProductDe ... ode=100621 or the WDD SE version drives), new RAM (128-bit DDR if your motherboard can support it... else you can settle for 32-bit RDRAM... or slower 32/64-bit DDR) or just a new mobo/ ram combo. I suggest you just hold off till early 2003 for Nforce 2 since it supports duel ram paging (if you have like 2 512mb chips) and such with 128-bit DDR.
a new GFX card can help with DECODING but only capture cards (gfx card if you have like an AllinWonder...) help with ENCODING.
also (though it's pointless over the short run...) suggest you pick up one of those new SB Audigy 2's in October. You may not encode anything over 48khz for a bit (or even 44khz for those ppl who aren't even SB compatible yet...) but the 96khz option can be really nice for HQ cuts (yes there are other 96khz cards out, Terratech 6-fire I believe among others, but the SB Audigy 2 will be huge!)