Hi chaps

My recent purchased mid -2010 i3 21.5" iMac is fairly pedestrian now I am loading it up with stuff. Time machine and other backups are really slowing it down. It has 4gb memory.

a) what is the best/cheapest place to buy?
b) will I be able to fit it myself or might I really screw things up
c) Is there any point in getting just an extra 2GB?

I'm not actually using anything particularly memory hungry.

Cheers