Nice, grats on finding the problem. Before spending money on a new case (if you really want it go ahead), you can try to fix the airflow of the one you have. Having a cheap case doesn't mean it can't be good, i have my pc on a very cheap case (have a Phenom II 955 and a ATI HD7950) and everything is under 50° on heavy load. It's all about how you make the air flow throw the case.

This is a typical airflow design (except from the top part):



The main idea is to make a tunnel effect, where cool air enter in the front and hot air get out by the back/side. If you want you can post a picture of your pc so i can tell you what you have wrong.

Ah, another thing, make sure everything is clean, not just the GPU/CPU cooler, check the front panel where the air enter and the fans on the case.