Goautodial is Vicidial under the hood. Adding new servers is easier with Vicibox as it handles the interconnects automagically. Upgrading is also easier in a Vicibox installed system. However, upgrading is possible with Goautodial ... it just wipes out the customized goauto pages and leaves you with vanilla Vicidial (which is a good thing, IMHO). Mixing old Goauto and new Vicibox servers is not a problem.
If you intend to build a Single System with over 150 seat capacity, you likely should engage a consultant. Over 500? You're IQ is in question if you don't engage a consultant.
600 to 1000 seats, however, is a bit vague. I have systems with 600 seats on a single server. Because it's rare to have more than 20 agents actually logged in at the same time, much less actually dialing outbound.
So for capacity it's important to know the total MAXIMUM of simultaneous logged in agents, outbound autodialed calls, inbound calls in queue and anything else that would be using load "right now" (AMD? g729? Survey?)
If you want to do this without a consultant, your best bet is to always have at least one (possibly two) servers of each role (Dialer or Web) ready for expansion. One "powered up and ready" and one "still in the box". Do not combine roles on servers. One server is DB. One server is Reports (replicated DB), and several servers each for Web and Dialer. You can certainly install Web and Dialer on all servers, just don't use both on the same server except in unusual situations. One role per server gives you the best bang for your buck on each server.
DB server should be RAID10 with fast drives and a solid RAID card (The Vicidial Group strongly recommends LSI MegaRAID). This also applies to the replication server so it can keep up with the DB server. Plus lots of memory and cores as DB servers love those.
Be very sure the interconnects on the servers are gigabit switches and there's nothing complex in the local network routing (don't bounce through several switches/routers between the servers).
Have a Vicidial Professional familiar with your system so that when you exceed capacity they can tell you which barrier you have hit by looking at the system they already have access to (as opposed to beginning the search or allowing access while you are experiencing a "bad day", that's just poor planning).