I'm coming down with something, so making this quick.
I think it is 1GB of ram for 12 (or 24) people, the rest is up to your ping and upload speed.
Speedtest.net - The Global Broadband Speed Test (You want about 50-60ms ping and Mb's of upload speed). If you want about 100 people on a server, you will need a REALLY stable connection, if you want 24 people on your server, not so much trouble.
Also, Tutorials/Setting up a server - Minecraft Wiki
Dual channel RAM:
Nice illustration on page 3.
Depends how many monitors you are using, most setups are fine with 1GB.
If you want something like 4-6, then start worrying about VRAM.
Any of the newer motherboards with 4 memory slots should be able to handle 16GB+.
Not sure if you're looking at Intel or AMD, so post up what you find and we'll let you know.
Please read through the thread thoroughly before responding.
Nobody wants to read a wall of text, formatting is your friend.
i5 3570k | 8GB G.Skill 1600MHz | R9 290 | SeaSonic 750X