My 20" iMac and it is old, was running slow with lots of the famous spinning Beach Ball o' Doom
I was able to replace the RAM chip with 2-PC2-5300 2 GB RAM Chips and it took me all of about 15 mins. If I had known about this site and all the great guides I would have done it a lot sooner.
Don't be afraid to tackle repair jobs like this, Read the guides, Then read the guide the again. And just to be safe read it one more time. Be organized - YOU CAN DO THIS>