150 agents? Cool. Get that single server method locked down tight and then look at having two web servers and several dialers.
You'll also very likely need to be sure you have gigabit switches to the servers so they can communicate with each other quickly and be sure they have gigabit to the rest of the offices (beware of 10/100 switch bottlenecks ... get at least one gigabit port on each switch to communicate with the servers!).
There's actually a good amount of "Enterprise" readiness you may want to address before you pass the 100 agent mark to avoid downtime. Or grow slowly so you can fix it all as you go
Web servers will need to increase capacity, all servers will need to have excess processes killed (no need to log unless you're troubleshooting). Hard drives should be cleaned nightly. Log tables archived and pruned. Lots of stuff.
Happy Hunting!