10 million users on a pair of gaming PCs is ridiculous. What's your product, a website that tells the current time?

How many requests do you expect users actually do? Especially if you're serving a B2B market; not everything is centered around addiction/"engagement". My 8 year old PC can do over 10k page requests/second for a reddit or myspace clone (without getting into caching). A modern high end gaming PC should be around 10x more capable (in terms of both CPU and storage IOPS). The limit in terms of needing to upgrade to "unusual" hardware for a PC would likely be the NIC. Networking is one place where typical consumer gear is stuck in 2005.

Webapps might make it hard to tell, but a modern computer (or even an old computer like mine) is mindbogglingly fast.