A good cleaning with a feather duster (whilst the computer is unplugged, obviously!!) can do wonders for an aging PC. Dust gradually builds up inside, and all the extra material settling onto the electronic components helps to generate lots of heat. Dust away gently over the various cards and around the fans and you may be suprised at the result.
If this doesn't help, a 'drastic' tactic is to run the PC with the side of the case removed. This can be risky if you're clumsy, or the room is messy, or you have lots of objects around your desk. Basically you need to be absolutely sure that nothing is going to fall inside your PC, and you are not going to touch it with your arms or feet or anything. Particularly dangerous are chocolate wrappers... but if you
are careful, this can be a great way to keep everything cool. The computer is absolutely fine to be running with all it's insides exposed. You can even point an electric fan into the machine for a boost of extra coolness (I personally used to do this before I got my Alienware).
Or, if you have some spare cash, why not invest in some liquid cooling? Absolutely the best solution, but do make sure to buy from a reliable brand and get it fitted by qualified technicians.