my 600 duron is now running at 1016 (9.5*107) at 38C with a global win fop 38 and arctic silver. The global win made the biggest difference for me, although the arctic silver gave me about 3C. navi is right too, you have to keep cool air circulating through your case to keep temps down.
However, my $5 stock h/s fan got me to 900 no problem, and I've spent about $60 for the last 100 mhz. ($25 for fop38, $13 for arctic silver,$10 for loctite, $10 for another case fan)Thats more than a new duron 600 processor! But I had to try for 1 ghz.