How Do I Upgrade PC2700 DDR RAM?

I have an old 2006 PC from Computer Builders Warehouse and I was looking to upgrade my RAM since I actually opened it up for the first time ever to look at what RAM and HDD it had. I saw it has only one 512 MB PC2700 DDR RAM which is a really extremely low amount of RAM. If I were to upgrade, what RAM do I need to buy? Do I need to stick with PC2700 DDR or can I go with PC3200, or a better RAM?