My work Mac is a 2x2GHz PowerMac
p) with 512MB RAM
(). If running just one or two apps, it's great, but if running with Mail and Safari (in particular) open in background, Xcode, Console, Interface Builder running it's a bit of a pig.
With the current OS, and in particular on Intel Macs with Rosetta, I wouldn't really consider 512 to be adequate for any grade of user, unless you have a lot of patience.
It's a pity Apple don't:
a) Default to a larger amount of RAM. If a user is confident they can manage with less, then let them choose a smaller amount.
or
b) Don't provide any default, and force the user to choose an amount of RAM.