Multiple rails app hosted on single server

What’s everyone favorite way to do this these days? I have nginx with 4
mongrel processes now which is great for the performance of this 1 rails
app I’m running. But to add another rails app to the box would require
4 more mongrels. Each mongrel takes like 20 MB of ram for the ruby VM.
Is there a way to run 4 mongrels total but have each mongrel switch
on-the-fly to the needed rails app?

That way I could run X # of rails apps but always keep the # of mongrels
steady at 4.

Or, is it better to use fastcgi and forget about mongrels if I want
multiple rails apps?