Hope someone can help me with this...

I have a Dell XPS M1530 and they are well known for overheating. It is usually fine but will overheat when I play games such as The Sims 3. I have a temperature monitor and apparently my aux (whatever that is) will reach 80 degrees C and higher when playing it.

So I bought a laptop cooler, and it seems to work fine when I'm not playing any games, but my laptop acts as if it weren't there when playing the game.

I'm just wondering what else I can do? I've cleaned the fan out but I have heard that it is a good idea to get thermal paste reapplied. Would you recommend this? My laptop is a year old.

Do you use your laptop sit flat on a desk or is it titled to an angle? An angle allows better air circulation and heat deflection, so if it isn't already, place something to ensure your laptop is titled, it might feel a bit weird at first for typing though, aluminium ones are good as aluminium is heat sink.
You can check to see if your fans are working properly, sometimes you can download diagnostic software direct from the company, e.g Dell or you can download another yourself.
You could check your BIOS settings/get a BIOS update from Dell, they can be a bit annoying and slow to do though on windows.