Venting....

I was thinking if i cut some slits(like they have on school locker doors) in the top of my case would that help cool it since heat risesand it would all be trapped at the top?

the best place for slits will be at the bottom to let cool air in. The powersupply will have a fan to exhaust hot air out the top of the case on the back. To increase case cooling i'd add another fan to the back of your case and bundel up any wires.

That could work and it would be a passive (re: low noise!) method of increasing ventialtion and improving temps....and here is the but!

BUT- airflow is tricky and does not always flow just where you think it will. If you have active cooling (fans) for most of your airflow those vents may change a pattern that is already working.

Most common with overclockers is forcing airflow where we want it with fans and many use the same concept with a fan- called a blowhole (fan is exhausting air out the top of the case.)

I generally try to test case ventilation mods with cardboard before committing myself by cutting a case up; if you can doing that may save you headaches caused by having a hacked up case

All that said, I did build one system that uses only exhaust fans at 7 volts and its cooling performance is quite good. Passive cooling tricks can work very well but they are harder to figure out than forced airflow.

No point in cutting holes unless you're wanting to stick a fan there.