From my side i would say setup a single server with a lot of RAM ( min 48 GB ) and min a Quad Core with 64bit operating system and put the Database on the second System. ( We have our system that serves about 1000 users as a vmware machine with 6 cores and 64 GB Ram -- which are a little bit oversized :-) )
Setting up a cluster also means that you allways have to care about multiple servers while doing os updates, updates on your system and so on.
You can also have a backgroundmethod server in a single server instance and also start multiple foreground methodservers.
If you configure the mod_jk status page you can even put a single methodserver out of duty in order to restart it without disturbing the current users.
As far as i have seen over the past years load balancing between multiple physical Servers wasn't really needed as the performance bottleneck in Windchill allways was the database but never the cpu / the methodservers.
The Methodservers will only cause problems if one programs an infinite loop and CPU goes up to 100% because of this bug.
Read this regarding performance: