Mac OS (really the *nix family of operating systems) is no different than other Operating Systems. Of course it will run faster with more RAM, and it will cache file systems up to the limits of physical memory.
In my experience, *nix systems run faster on a given memory footprint than MS stuff.
For most people, anything in the core i Series is "fast enough", and I realize the white MacBooks are limited to Core 2 Duos. They aren't bad, especially running Mac OS on them. The SSD is the key there.
Of course it is, it's far easier to spend other people's money than your own after all.