Mac :: Aluminium Macbook, Memory, Heat, Speed, Response Time Options?

I have got the 2.4ghz aluminium macbook, and I upgraded the ram to 4GB about a 2 months back. I have always felt that my macbook is slower now than before the upgrade(which is strange), but recently the situation became worst. My macbook would randomly heats up very quickly, iStats pro would show me that i only have 1-10MB of free memory, my fan would be running on 6200rpm. response rate would be much slower, and circle of rainbow also occurs much more often.I get panic whenever this happens(i have only got my macbook for 5-months), so when that happens i usually try to quit those application that i am not using like mail, safari, iwork 09 and etc. It didn't really help much(or maybe not at all) until at some point(around about 30-45mins) the systems automatically cools down, reduce fan speed and etc but similar problem would happen again in 30mins.