In the recent past I experienced the same thing when working on bigger more intensive projects, especially when scrubbing. I upgraded from 4GB to 8GB RAM and haven’t had the problem since.
You may also want to check and see if the RAM you have now is “OK”. Go to the “System Profiler” and under the hardware tab select “Memory”. It will let you know what RAM you have and in witch slots and also if it’s “OK”.
That said, if the budget is tight, then I would be sure to rule everthing else out before purchasing more RAM.