I have given up on using PC since when I compared a Pentium 4 1.8GHz against a RB493, and surprisingly enough the RB493 was performing better. Thing is, Routerboards have CPU designed for routing purposes, while computers for other things. With a computer you need to by extra network cards, RouterOS License and it consumes more power than the RB110AHx2. Instead, with a RB110AHx2 you already have those network cards, Level6 ROS, and a better backup power plan.
I also have a customer, who once used an HP workstation Pentium 4 3.4GHz, and has switched now to two RB1200. The situation is way better now than before.
There is one more thing. A used PC does have a used HDD. The license of the RouterOS is attached to the HDD. If the HDD fails after sometime you could have some trouble on getting back the license.
Don't crash the ambulance, whatever you do