How much RAM can I add to my macbook I have a 2008 macbook with a 2.1 GHz Intel Core 2 Duo.

I am in the process of upgrading to Lion.

I currently have 2GB 667 MHz DDR2 SDRAM

What's the maximum amount of RAM I can put in and it have an effect?
everymac.com is an awesome source for this kind of information. They've got detailed specs for all Macs.*

Yours is listed as supporting a maximum of 6GB of RAM. Technically, Apple says that this computer only supports 4, but it has been proven to actually support 6:


  Apple officially supports a maximum of 4 GB of RAM but third-parties
  have been able to upgrade the system to 6 GB of RAM using one 2 GB and
  one 4 GB memory module.


You can buy a 6GB kit from OWC, a great company, for $120.

If you use a lot of apps and regularly run up against the edge of your available memory, everything you add will make a difference; the computer can take advantage of all of it. However, since you've currently got 2, you should consider if 4 will be enough or if you want all 6.

*In additon to Everymac, I recommend the app Mactracker for Mac and iOS (free). It has detailed info and specs for all Mac models.