When I bought my wife's refurbished MacBook I got it with the smallest amount of RAM I could get, in this case 1GB. The reason is that Apple charges a lot more for memory than what you can buy from aftermarket sellers like Memory America or Other World Computing, both great resource for Mac memory and hard drives.
I purchased a 2GB memory upgrade from OWC for $41, which included the cost of shipping. For perspective if you bump the memory on a MacBook from Apple up from 1GB to 2GB there is a $100 charge for it as of this writing. The nice thing about the OWC site is that they will help you identify exactly which memory works for your model of MacBook. If you have another good resource for Mac memory please add it to the comments below.
While 1GB of memory is adequate to run OS X smoothly on a MacBook I think it's important to have as much memory as you are comfortable putting in your machine. Some applications— especially Firefox, my primary web browser—leak memory pretty badly and the longer you run those applications the lower your performance can get. As I compose this post on my Mac Pro my little iStat memory counter tells me that Firefox is using 855MB of memory. Ugh - time to restart Firefox.
The only downside to buying the memory from someone other than Apple is that you have to install it yourself or take it somewhere and have them install it for you. Installing memory on a MacBook is very simple and the only tools you need are a coin and a #0 phillips screwdriver.
While I was preparing my wife's machine I decided to put together a quick video to show how easy it is:
All told it takes about 3 minutes to do and can save you quite a bit of money by doing it yourself.