Upgrading a 2006 MacBook Pro to 3 GB Ram, is this a good idea?
You might know already the discussion here:
https://discussions.apple.com/message/8053632#8053632

If you are actually limited by RAM, then I would not assume that the performance impact of a mixed 1GB/2GB setup would change much. Swapping to disk is so slow, it's hardly compensated by RAM timing issues.

However, first make sure that RAM is actually your issue.