Actually, I'd reccomend half that amount (128MB). I got 200MB and never use more than 1/3. Dunno if the installer demands more though.
Then again, I didn't install all the junk that was on the OS39 cd to my bootdisk...
I have to reccomend that you install KingCon (Shell replacement) and Toolmanager (program launcher) both available at Aminet, instead of those supplied with 3.9. They really suck.
And oh, if you want some slight notion of speed:
C:SetPatch QUIET SKIPROMUPDATES=exec.library
(edit)
Me stupid; his h/w has a future with OS4. Go for 256MB.
(/edit)