First thing to check is how tight the existing heat sink is mounted to your GPU. This is the single-most important factor in effective cooling. If it's not tight enough, your cooling will really suffer. The catch is that you do NOT want to over-tighten. If they already seem tightened down, you might want to try another 1/2 turn or so.
Other than that, Arctic Silver will help and usually will drop your temps around 2 deg Celsius or more from stock thermal compounds.
Check for dust buildup as well, clean the heatsink with compressed air and make sure your case has good air circulation.
Also, this section of this review might have some info that could help you out: