A couple of answers that should have been provided a while ago (I hope you didn't take your PC in because of the lackluster replies! They can be helpful, but they are damned expensive, and more often than not, all they mange to do to set things right is a tiny bit of maintenance.)
Total system memory in-use can be checked via the task manager (ctrl,alt,delete), near the bottom or on the performance tab. It will list physical and virtual memory (swap/page file, a section of predetermined HDD space set aside to act as a slower version of RAM). 2Gb's of RAM is fine for now, but the best way to determine when to upgrade RAM is when you no longer have enough to keep up with your apps! It really is a per basis decision.
General maintenance is a must. I don't mean to berate, but the dust in the fan/heatsink was probably your problem, and those
porn-stealing "techs" just took you for a ride.

I'll admit some CPU heatsinks can be quite bitchy, to remove the fan requires no more than a screw driver, but you shouldn't be afraid of breaking anything (long as the computers off and unplugged! And you ground yourself to remove any ESD (static electricity)). Just examine the confounded contraption and work out how to unclasp it. You don't even need to remove the heatsink to clean it most the time, you can use a can of compressed air, or lacking that some papertowels and q-tips, maybe some Isopropyl alchohol. This job doesn't require finesse..
Any other questions? Any at all, regarding anything, We're happy to answer.