What kind of motherboard do you have it on? If you have one that will allow you to select a 133 fsb speed, you stand a fair chance of it running at 600 mhz on a 133 fsb. It wouldn't be the fastest folder on the planet but it would still be able to turn out a fair amount of work. As far as what you could get for it, I don't think that you could get much more than $50 for the proc and cpu together because it is so old and slow. This is only my opinion though and you might want to hear a few more before trying to post it for sale in the classifieds.
As far as the mobo/duron is concerned, that is an excellent idea. Newegg had some refurb shuttle AK31 boards for well less than $50 the other day shipped and they have a 30 day warrantee. One of those and a duron for around $100 would make a good folding rig.