If you have OSX 10.4.11, that is above the minimum, so you should be fine. There is no way around it, you have to buy Leopard for 10.5.
Buy the same type of memory; as far as I know only one type will work based on your model iMac. Before you buy the memory, check to see what you have, ie. number of slot(s), size of memory in slot(s). You may have to replace what's there to increase your memory, not just add to it.
And the memory is easy to install if you have any minor skill with a screwdriver. Just follow the directions.