My MacBook is about a year old now and I've read in quite a few places that they could be upgraded to around 3.3GB of memory for this specific model. Unlike the latest MacBook which can "as advertised" accommodate 4GB of memory, the theory was that the Santa Rosa chipset can address all 4GB. It could very well be that technically my MacBook is only using ~3.3GB of that memory but the performance has been significant for a very cheap stick of RAM. I highly recommend Crucial.
I think this upgrade will have to hold me for some time. In general it has really helped my uptime. Odd as that is I think working with only 2GB of memory caused a lot of IO swap and memory corruption. Especially when stopping and starting so many different mongrels for local Rails development. As of this writing I have not found an issue that I have had to reboot for... uptime 10 days and going strong.
To keep my uptime going strong, I just purchased an app called iFreeMem today after reading about it on TUAW. Because I use PhotoShop ever now and then it really helped having this utility free up some memory that it would not release. So far it is working great!