It's the old chicken and the egg problem.
Before you can determine what type of server you need,
you really need to have a working application or at least a test
application that you can 'spool' up and test with a dozen, 50, 100, 500 users
and see what sort of load is placed on the box.
We run game servers at my place of work and most of them are Dell boxes
along the lines of the Dell Poweredge 650. P4 2.8Ghz with 1.5GB of Ram.
Drive space is minimal as the boxes only host the games exclusively.
Bandwidth throughput on these is a constant 20Kb/sec. thats approximately a
low grade ADSL connection. Thats with probably 30-50 users max at any one time.
So the games servers aren't heavily utilised, but they certainly have a hefty requirement
in terms of setup cost and bandwidth.
If you are taking scalable in terms of hundreds of users your requirements would rise accordingly. Not only that, but how would you even market your product to the masses?
Thats why I say, write a test app first, try it out and see how it all goes.
If you get past the test app phase, then maybe you are ready to take things a step further.
Everything happens for a reason