Which model are you referring to? Do you already have the MBP or are you considering a CTO from Apple? Most will advise NOT buying RAM from Apple as they charge a large premium for something you can do yourself.
You can add RAM to MBPs, but sometimes it can be cost prohibitive. My MBP came with 4 GB and to max it at 6 GB, it would cost over $300 to replace a 2 GB chip for a 4 GB chip. It isn't worth it to ME.
The older MacBook Pro models cannot utilize 4 or more GB RAM. For instance, my early 2007 17" MBP can use just 3 GB and my earlier MBP (2006) could only use 2 GB. It's a good idea to check the exact specs on the machine before doing anything with attempting to update the RAM. The 17" MBPs that came out later in 2007 are able to utilize 4 GB. It's important to be sure whether or not you can upgrade the RAM in any used Mac if that's going to be significant to you.
My 17" MBP is the Core Duo 2 2.33 GHz with the ATI Radeon X1600; it was replaced by the 17" MBP Core Duo 2 2.4 GHz, which has the Nvidia graphics card. My model can only accept 3 GB RAM, whereas the 2.4 GHz one can accept 4 GB RAM.
So check to see which model is being offered and check the specs before buying!