I'm hoping you get plenty of answers, as I'll be doing the same thing shortly! One thing I know, if you have 16Gb of ram, you need a 64 bit OS.
Windows will be the easiest OS to host it on if you aren't familiar with Linux. All you need for CSS is the SDK and any plugins you want to include.
My internet is fast enough to host games servers though. I just hosted a 3 man CoH, and I know I can run a UT2k4 server for 6 people. But yes, UK internet in general sucks!
easiest by far is to use windows if your a general windows user, you can setup multiple games as long as you set different ports in the execution settings for example ./srcds_run +port 27015 and ./srcds_run +port 27055 Not hard to get admin mods installed and css is the easiest to get started with. loads of tutorials out there. good luck and let us know how you get on. If i were you i would run a server per core as not many server binaries are multi-threaded... mind it's been 6 months since i set one up last so it may have changed recently
Linux can be tricky. Not all games come with server binaries for the OS. Windows is really easy, especially for CSS and other source games, you just use steam and the dedicated servers are listed in the tools section.
I prefer Linux for game servers, as its generally more stable (my best mate runs a big UK GSP which has been going since 2003) but the lack of binaries for some games is a real problem. CSS and most other HL engine stuff work fine though. Whether you use Windows or Linux however, you need a distinct steam folder for each unique server, and then as WarrenJ says above launch each with a different port like: Alternatively if you're LAN only, launch with a unique IP which would need to be bound to the server in the usual manner, eg via network adaptor settings in control panel, or rc.conf in freebsd, /etc/networking/interfaces in debian/ubuntu, etc... On a box of your spec, as long as its dedicated to gaming, you should be able to run around 100 slots of 100 tick more than fine, and if you're using 66 tick, maybe upto 200 slots. That assumes 90% occupation of the slots however.