Upgrading RAM on early 2007 Macbook pro, 2.16GHz

I have an early 2007 Macbook Pro, 2.16GHz Intel Cor2Duo with 1GB 667MHz RAM. I have just ordered the Snow Leopard update & would also like to upgrade my RAM.

A rep at the Apple Store checked my serial number and recommended I upgrade to 4GB RAM at a cost of about $200. http://store.apple.com/us/product/MA940G/B

When I search for RAM on other websites I can find it for much less. Is there anything I should be aware of or any reason why NOT to go with the less expensive options?

When I searched for RAM at the following website,

http://eshop.macsales.com/shop/memory/MacBook/Pro/Core2/

it said this about the maximum RAM.

*MacBook Pro 15" 2.16GHz, 2.33Ghz and 17" 2.33GHz models Apple factory specification maximum 2.0GB, OWC tested to support 3.0GB maximum RAM. 4.0GB can be installed, but only 3.0GB will be addressed.

If I purchase 4GB am I only actually getting 3GB due to my computer's limitations?