If you don't want to read all of this, the TL;DR is below...
Background: We're a medium-sized eCommerce store which operates parallel to several brick and mortar stores under the same brand. We're switching from an older open source shopping cart to one of todays eCom beasts: Magento.
We have an existing relationship with an ISP who will do our hosting. They colo our existing web server for our current site, and have suggested that we use their new-ish virtualized server environment (read: VMs) for the launch of our new system.
I am the in-house developer and have been working with these VMs for a few months now, but keep encountering performance problems. I've raised my concerns multiple times and finally got some people to listen, but now I've been asked to help define some "hard numbers" or metrics/KPIs that will tell us whether the servers will perform fast enough for us come launch time.
Questions / TL;DR: How do you know what to purchase for servers for a new project? Is it driven by gut instinct/experience or do you have some sort of formula you use?
I imagine the most general answer is "it depends" on the platform, application, expected usage, etc. But that's exactly what I'm looking for? What do I need to know? Where does it all fit into the calculations? How did I get into this mess???