Resources usage in dedicated server issue

I'm trying to make a Counter Strike:Source server with following properties:

• Maximum players: 15
• CSSMath plugin
• Fully voice enabled

With the above properties, how much resources (Bandwidth, RAM, CPU, etc) are needed? and how can I calculate the resources needed?

It's hard to determine the resources for Bandwidth as you're going to have players joining and dropping at random intervals.

For a single TF2 server running 24 players, expect to need at least a 2.4GHz processor (AMD or newer-generation Intel, old Intels like P4s won't be fast enough), and 1GB of RAM.

--TheMG at SteamPowered Forums (link also contains original source for bandwidth calculation.)

TF2 is relevant because it is essentially the same engine running. I don't know much about the CSSMath plugin, so you're on your own there but from the experience I have with servers and plugins, it shouldn't be too taxing.