The PLC has built in "work" memory that can't be expanded, which is used like RAM in your PC. The "load" memory is expandable (whichever memory card you select), and works similarly to the hard drive of your computer. When you download from Portal to the 1500, it downloads to the Load memory. Then it copies things into work memory when it's time to run the program.
The recommendation I've heard is to buy a memory card at least twice as big as you actually need, because if you need to download the whole project at once for some reason, it needs room on the card for all the new blocks before it can delete the old project data.