You can't go wrong with a dual core CPU, though it would be a shame to orphan a perfectly good socket 939 processor. Are you going to sell it or build another system around it?
I use a pretty mediocre Dell P4 3.2 GHz system at the office and it runs Vista well, though the "Aero" interface is disabled because the onboard video is weak. As long as you have at least a gig of RAM and a half decent 128 MB video card, Vista should run with all of the bells and whistles enabled. It's actually a pretty nice OS, once you tweak it.
I had read the article but could not find the chart a second time. Hmmm. sounds like a tough question to get rid of a perfectly good AMD 3000+ to see if there are any changes with Vista performance on a 4800+. (I don't play games and have a Geforce 6600)
Currently running 2 gigs of memory... this may be the last upgrade for the the gigabyte K8n9-f motherboard.