honestly, i run a minecraft 1.5.2 server and 1gb of ram is enough (i mean a lot more than needed for around 20 players, it's never using more than 5-600mb of ram)
So 8gb or 16gb config is a lot and put more cash on a good cpu as Minecraft is java based (Java don't use GPU, only CPU power so no need for a really high-end GPU but a higher end CPU will help a lot)
I would suggest a 3570k or a fx-6300/fx-8350 to have plenty of power (pretty much overkill just for minecraft but for running other games and still run your minecraft server, you'll need something beefy (well not that much but a minimum of 1 good core in permanence...)
Do you need an entire PC or do you have some parts like PSU, case, OS, etc. ??