You can have up to 2x1gig sticks. You can get 533mhz but it will auto cut it to 400mhz, some of the best bang for the buck RAM right now is this
http://www.newegg.com/Product/Produc...82E16820144183
Also depending on your processor model, there is an easy pin mod that will bump your bus from 400mhz to 533, not only increasing your processor performance by leaps and bounds, but also lets you fully utilize the 533 RAM if you do get it.
Hope this helps a little.