When a computer is said to be able to take up to 2Gb (Actual) RAM - 1Gb (Apple) RAM, what does that mean?

What are you on about? haha

A Mac can take as much RAM as you can stuff it with.

In the past, Apple has said certain models can only take, say 512mb of RAM. This was usually when 256mb sticks were the biggest you could get (so you'd use 2 of them).
When bigger sticks of RAM came out later, it was discovered that they also work.
This is where the Actual v Apple thing comes from.

Thanks mate

Adding to Dangelovich's post,
The Actual Max usually represents the biggest chip size the particular RAM was made in, x no of slots in the machine.
Using the same example, 1024MB might be the max, because 512MB was the biggest stick size made for the particular RAM, before it was superceded. The Apple Max figure relates to the size of RAM chip available during the period the machine was a current model, and hence was sold with that RAM configuration.

The old iMacs are a good example.
Originally my Rev.D G3 333 MHz was sold with PC66 144 pin SODIMM RAM, configured with 32MB std and a vacant slot.
You could stick a 64MB in the empty slot, or 2x64 replacing the 32 as well. - Apples Max 128MB.
These machines are now capable of holding 2x256MB PC100 144 pin SODIMM RAM, 4x the original max configuration. The RAM speed was increased before the chip configuration itself was superceded. There is no 512MB chip of this type.

Note: some machines have a chip soldered on, eg the PB 12 inch 867MHz, so only one slot is readily upgradeable, not both.