50 Mb per developer sounds like a good number... If you're not doing anything intensive you can push a little further... Our little club server runs OpenACS in a P133 with 64 Mb RAM and slow IDE drives (hey, it was built with donated parts - VA promised us a server and never delivered), gets 500-1000 hits/day and the swap is very small.
This little server also does lots of FTP serving, rsync, lists, etc. Whenever we tell what our server does and its specs to NT people they simply tell us we are liars 😊
