Ok here are the probable culprits:
Lack of power
Out of date driver
Botnet leaching your processing power.
If its overheating, download speedfan to check your temperatures (just google it and it should be one of the first links), an average temperature for a GPU is anywhere between 40 and 65 degrees celcius. If its above that, then its time to clean out your computer with compressed air and invest in some thermal paste.
It could also be that not enough power is reaching your GPU, post what wattage your power supply is if known, if its 500W or higher then it should be fine, and you could probably get away with around 400W.
Go to ATI's website and download the latest driver for your graphics card, then test to see if it works better.
Alot (and i mean ALOT) of computers have botnets on them which leech alot of processing power and internet speed. If you close all non-vital programs down (like MSN, Skype, Winamp etc...), and Speedfan detects alot of CPU activity, then youve probably got a botnet on your computer. The only way to 100% get rid of a botnet is to save all your files to an external hard drive and reformat.
The last thing i can think of is that your GPU simply cannot run those games and youll need to buy a better one.
Edited by Welshhobo, 11 September 2010 - 06:21 AM.