wait..... you actually have open you pc to figure that out...... but most likely you have two cards of 512 each.
Open the System unit (CPU) and look at the RAM slots on motherboard. They look like this:
In this picture, ram is used in ALL the slots.
From here on, you can figure out how much ram you got on each slot. If you got only one ram stick, it means you have one Gb ram with all other slots open. If there are two ram sticks, it means you have two 512 Mb rams combining up to one Gb