You can use the same thermal compound on the GPU and CPU, reommended ones would be Arctic silver 5 or Zalmans thermal Grease.
Temp reductions will generally be a couple of degrees celsius but if you are replacing previously poorly applied compound then its could be more.
To remove already exisitng compound use a high percentage alcohol. I found the cheapest way was to go to a pharmacy and ask for some Surgical Spirit. Very cheap and does the job perfectly. Make sure you use a lint free cloth to remove it too.