well since august, my system has grown because of multiboxing... I started as 2 then 3 then 5 clients... I needed more screen real estate (@3) so a 24" monitor had to be bought, then for running 5 I upgraded my MB and memory (as well as the OS)

so yes it evolves over time.

I think the only addon that would cripple my gameplay would be the loss of trinity. losing a couple of hundred macros (across all 5 chars) for each team really would cause me some problems