while i've never deployed that large (several members on the forums have and we'll have to wait for one of them to chime in) --
I'd think the most used resource will end up being RAM -- like with most enterprise applications. That many users concurrently will be a lot of memory overhead for the system to keep track of everything. I'd think CPU would play a role too, but then again, there are web servers running out there with single core's and still serve up to thousands of visitors -- just may take a little longer to respond to requests and such. take this as a grain of salt though until someone with large deployment experience chimes in 