Thank you to both of you.
I have 3 GB of RAM. It should be enough, as this also happens when I'm doing something simple like surfing the webs.
As for the overheading, I've checked with "Core Temp". It says my cores are only around 30 degrees celcius.
I think my fan is working, but I might check it out some time.
I've been thinking about changing my CPU for quite some time now.
It's getting old, and I need something new the next time I buy a game, I think.
Again, thanks for replying