Actually, to play DivX smoothly, a Pentium200 will suffice
*IF* you get one of the new decoder card. Unfortunatley, one of these puppies runs ~90$. The good thing, is that it will decode MPEG-2 for you too, so it's like a combo MPEG-2 and MPEG-4 decoder card.
Go to
toms as he covers it in one of his articles. It might be significantly cheaper (And I know a hell of alot more "Headache saving") than getting a new motherboard and processor and RAM. If you're lookig for a system upgrade, do a system upgrade.
If you actually want to keep this system in it's present form, you'll definatley need the Sigma designs decoder. And i'd highly reccomend it. It's a nice card.