Dashboard is pretty cool, but it eats a lot of memory. If you want to disable it, execute this at a command prompt:

defaults write com.apple.dashboard mcx-disabled -boolean YES

I did this but I’m also going to try to upgrade my PowerBook from 512 MB to 1024 MB, because a lot of OS X processes use a lot of memory and I don’t think that 512 MB is enough for a power user.