How come some of my video games crash after a few minutes of play but others do not?

Q: I am having issue’s with my comp. I supose, I bought a few games (Rage, Dishonored, Assassins Creed 3 ect ext) anyways, for some reason they boot me out of the game every few minutes, but when I play my other games it doesn’t. I don’t know how to fix this issue, rather I need to get more RAM and a Video Card (and which would give me the best for the money)((I’m building this comp into a very descent gaming comp))

I have:

Windows 7,
AMD Athlon 64 64 x2 Dual Core pross 6000+ 3.10GHZ,
8GB RAM
64 bit OS
750w PSU
NVidia GeForce GTX 550 Ti