You maybe lucky and your current system case has an empty mounting(s) waiting for that extra fan. -OR- you can install a buss type fan mounted in a card slot and usually exhausts the heat. The quick solution to prove you have a heat problem is to remove side case cover and use a household fan to into the case. Check at that time for any dust/dirt build-up and blow it out. The best solution if accplicable is to use a new system case with your old mtrbd. that has already better fan mounting, etc. plus be bigger in size as well. it also helps to route harness and wiring to help in air circulation of case. Check any "modding" website that discusses this issue in much detail.
good luck -----Willy