Well, you really don't want to gob up on the thermal paste on the CPU. The basic function thermal paste provides is to fill in the gaps between the HSF and the CPU die. Any more than is necessary to provide this function, and it will actually insulate the CPU from the HSF, causing all kinds of havoc with your thermal transfer and cooling. Arctic Silver has some pretty good instructions on their website for applying thermal paste, and they can be used in a pretty general sense.
http://www.arcticsilver.com/arctic_s...structions.htm
My thinking is if the system has been running awhile, you may want to just re-apply the thermal grease, as the old stuff may be getting kind of chalky. Just make sure you clean the HSF and the CPU die well with some Naptha (Zippo lighter fluid) , or Acetone (not nail polish remover, as it tends to have other stuff in it, like vitamin E with aloe, and I don't think your CPU needs to be moisturized), and then give it one last once over with rubbing alchohol to remove any left over residue.
Also, you may want to clean any dust out of the HSF, and make sure there's not alot of dust inside the system, maybe clogging the fan on the video card (common).