Expect an even greater difference between the stock thermal material and arctic silver 5.
Put 100% load on the computer and heat the sink up nice and good, take the sucker off. Scrape the thermal material off the heatsink while its still warm (comes off much easier like that).
Rub some rubbing alcohol on it. *edit* (The heatsink, Lidge.)
Get some steel wool (my favorite) or some fine grit sandpaper, and smooth the sucker out to a fine finish (use water with sandpaper if you decide to use that) *edit* (I'm also talking about the heatsink, Lidge). Be sure to use something to blow the small metal grit out of the heatsink (like compressed air... or a hairdryer...) if you use steel wool.
Then rub it down with more rubbing alcohol.
Then follow these instructions on putting AS5 on it:
http://www.arcticsilver.com/arctic_silv ... ctions.htm