Which RAM to BUY? Screenshot included

Please take a look at the image below.
Currently I have 2 GB of RAM spread out over 4 slots (4 x 512)
I would like to upgrade to 4GB, and possibly beyond in the future.

Can you help me figure out the specs of what o buy?
Do I need to buy 512mb RAM sticks, or could I just buy 2 x 1GB sticks?

thanks-
Eric

you dont have to get 4 512mb sticks you can get 2 1gb sticks

Crucial will give you many options.

http://www.crucial.com/store/listparts.aspx?model=Mac Pro (4-core Xeon 5400 Series)

Looks like you would want something like this:
1GB Module
or
512Mb Module

Macsales.com has great prices and a lifetime warranty. You can install any size.