Yes, the memory is user upgradeable. However, the standard 512 MB comes in the form of two SO-DIMMs. If you want to upgrade to 1 GB, you would need to purchase a 1 GB stick to replace one of the 256 MB factory sticks with. You would therefore end up with 1.25 GB of total memory.
Also, I would suggest ordering the memory from somewhere like Other World Computing, which has very good pricing and customer service. Memory from Apple is normally more expensive than comparable memory from other vendors.
I just bought two 1GB modules (Transcend) from Newegg.com for $170 including 2-Day shipping. The Transcend modules were highly rated at a website called Macbook Resource Site (http://www.wiredbynature.org/comp/mac/macbook/mods.php)
I'd reccomend buying matched pairs. The MacBook uses integrated graphics, which means it siphons off system RAM. Using matched pairs gives you dual-channel support, which is faster than using regular RAM.
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