Topic: Multiple apps on single server


I'm thinking about making small business by selling/renting e-shop and CMS
applications written in Rails. Now I'm trying to figure out, how many such
applications can be hosted on a single server (let's assume 2x Dual Core
Xeon 2.66 GHz, 4GB RAM). All libs in total shouldn't have more then 20-30MB, an average
application can be assumed to have maximum of 200 active users at one time.

Is there any way to figure it out? I'm basically trying to minimize a
running cost per application and I love Ruby and Rails too much to go back
and do PHP stuff, though I guess it's impossible to beat PHP in terms of
running cost. Can anybody help with this?