http://www.tomshardware.com/reviews/gaming-cpu-review-overclock,3106-6.html
Your CPU is currently 4th tier, but that's not necessarily bad. Given what the customers are using them for, & that your gamers aren't having any issues, you probably don't need to worry about a CPU upgrade. If you absolutely
must, realize that your board is apparenly only a Socket AM3 (
not Socket AM3+) (http://www.asus.com/Motherboards/M4A79XTD_EVO/specifications/), so the FX-series CPUs are out. If you can find some, however, you could upgrade to the 975/980 (quad) or even 1090T/1100T (hexa) Phenom IIs, which would put you up into 2nd tier. However, unless you're really seeing some slowdown you don't need it. And, as was pointed out, switching to Intel from AMD would involve switching motherboards, which makes it even more expensive.
RAM-wise, you should be fine. Too much past 1333MHz, & you start getting diminishing returns for the extra cost. Plus, dual sticks means you should be operating in dual-channel mode. However, if you wanted to, a small boost might be possible by doubling the RAM (4x4GB) on the PCs.
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
If you really want a boost, especially for your gamers, this is where you can do it. The current GPUs are 12th-tier, & probably could stand to be upgraded. Even going to AMD's R7 260X or nVidia's GTX 750Ti would boost you up to 9th-tier. The R7 is cheaper ($125 vs. $150), but the 750Ti uses less power (recommended 300W PSU vs. 500W), so whichever works best for your setup. If you have the cash, another possibility would be either the GTX 960 ($210, needs 450W PSU) or R9 280 ($216, needs 500W PSU); both cards are considered 6th-tier, & would be solid upgrades for some time.