RAM will only speed it up to a certain extent. Your processor will usually bottleneck long before your amount of memory.
Your processor affects everything you do.
It sounds like the processor may be overheating. When my processor used to overheat on an older computer, it would lag when I pulled the mouse across the screen. The heatsink might not have a good contact with the procesor or there might not be a good layer of thermal paste between the CPU and the processor. Not sure how comfortable you are with computers, but you can Google for guides on how to apply thermal paste to a CPU.
You can download SpeedFan to check the temperatures (Google).
