I just updated the memory in my iMac and Macbook. The physical process is fairly easy except the macbook needs a really really really small Phillips screw driver. I used one of the blades on a multi-tool but it was not ideal.
The performance increase with 2 Gigs of memory over 1 Gig is nice but most noticeable in the Parallels Virtual Machine. Before if you were running one of the Virtual Machines it pretty much locked up the rest of the machine. Now the rest of the machine is running just fine and the Operating System running in Parallels is very snappy.
Purchased the memory from Crucial. I was just poking around and looking at what the price would be to upgrade. When I realized it was really not that much I decided to go ahead and purchase the memory. Cost about $21 on the iMac and $46 on the Macbook. I had to replace both memory chips in the Macbook. The iMac only needed an additional 1 Gig chip.