Recently, my GPU and processor have been overheating. I have the Nvidia monitor and when i logged it, some of my games crash once the GPU gets to about 165-170 degrees Fahrenheit. This hasn't happened before, and i was wondering about modifying my manufacturer's Dell Dimension 2400 to have another slot for a fan. If that's not possible, a cheap case that would reduce temperature. Any suggestion would be helpful.

This hasn't happened before

So what changed? If it did not overheat before, it should not be overheating now - unless something changed. Typically, if no other changes, new problems of excessive heat is due to dirt and dust blanketing the components and blocking the vents and heat sinks. Failing fans, poor cable management, overclocking, and failing hardware can also cause over heating.

i was wondering about modifying my manufacturer's Dell Dimension 2400 to have another slot for a fan

There are slot type fans made specifically for helping with graphics card cooling - and they work. But they provide little help for CPU cooling, and tend to significantly add to the over all noise levels of the computer. I would make sure the case is free of heat trapping dust and inspect of additional fan mounting options.

If that's not possible, a cheap case that would reduce temperature. Any suggestion would be helpful.

Suggestions? Yes, don't get a cheap case. A good case will last you years through several upgrades. More importantly, it will safely house your computer, and provide adequate cooling options - such as multiple 120mm fan support. A good case is also easier to add and remove components - and will not shred the hide on your hands from the sheet metal edges. I recommend you avoid fancy lights - they do nothing for performance, add some heat, consume some power, and do nothing for performance (worth repeating). It is hard to go wrong with an Antec case.