Alright, I've been having a constant debate with my server tech about this.
As of right now, I have one web server that is a dual 5500 series (so about 2 generations back) with 64GB ram that has mysql disabled. For MySQL I have a primary update-only server, with two slave mysql servers for reading-only. So four servers total.
Even with this setup, I feel my site getting bogged down at times when traffic spikes. To top it off, we just launched our mobile apps and are expecting them to do a few million more mysql requests daily with a few million GD Library requests as well (so that portion will be on the web server).
I've been wanted to upgrade the web server to a dual e5-2620 series, but I'm told that I should instead remove cpanel/whm to free up resources and stick with the hardware I got. I feel like my server hardware is outdated, and that there must be traffic-handling advantages to moving up from 5500 series to e5-2600 series (2620 or 2640).