Using Multiple Servers For WPMU

I'm looking to implement the core system for my WPMU project so interested to see what people have and recommend. Specifically for a two server setup which can be expanded relatively easy in the future by introducing additional webservers for load balancing, and a dedicated memcached server.

I have 2x VPS servers ready to play with. I don't have to worry about mail as thats managed elsewhere along with everything other that will steal resources away from the resources of the core WPMU functions.

To begin with I've seen this setup suggested (my questions below are based on this) but are there any others worth mentioning?

Frontend - Nginx, WPMU,
Backend - MySQL using Multidb

Which will use more resources in general the more blogs you run, front or back?
How much resources does memcache use in comparison?
Memcache - Apart from being on its own server where is it best located, front or backend on a two server setup?
Do i need to worry about load balancing on the frontend for 50k blogs and if so if memcache is on frontend can database be load balanced using Nginx?

As for scaling, there will be a time when I will need to allow registrations from another geolocation, i.e. country. I intend to add additional local VPS's to help with load balancing and partition the database between datacenters and probably throw in a dedicated memcached server for the sake of it. Thats a different story though and a wee while away but tips for the future would be handy.

Thanks for your thoughts