Adding memory in an iMac

My iMac currently has two 512 MB cards installed. Can I replace just one of them with a 2GB card or do I have to use equal sized cards in each slot?
On the crucial.com website it indicates unequal size cards are okay, but I have also heard that the performance will suffer if they are not the same in both slots.

You should always install memory modules in matched pairs if possible. That will yield the best performance.

NOTE: Your iMac can accept 4 GB of memory (2 x 2 GB modules), however, only 3 GB will be addressed.