standard rule is 25 agents per Dialer. If you are working well with 35 agents per dialer, then you are doing well.
You should add two more dialers and a DB server with the same specs as the machine you have now.
OR
You could add four smaller (core2Quad, generic) "dialers" and one smaller "web server" to your existing system and continue to use the existing system as the DB server.
In any case I would recommend using Vicibox to do your Clustering (as it does so automatically!), but this will require upgrading your present system to the latest SVN release before adding new servers to the cluster (which is always the way to go, anyway).
Step 1: Upgrade the existing system to the latest svn (using the SVN:HowTo page in the Vicidial Wiki). Let this run for a couple days to be sure there are no upgrade related problems.
Step 2: Using Vicibox 4.0, add a 2nd "dialer" (which can actually be done without even shutting down the existing system!). Immediately move only 1 or 2 agents to it until it has proven to be reliable, then move as many as it will hold.
Step 3: Now that you have "add a cluster box" down, you may add a new dialer or web server at any moment ... Vicibox is Cool. And don't forget to thank Kumba.
(IE: Now you can expand at will ... any time your system seems heavily loaded, add a new machine ...)