Minimum requirements: (6-10 Players)
CPU : Intel Pentium 4 or AMD Athlon XP 2000+ and better
RAM : 1GB
HDD : 2GB empty space (5GB if you are doing frequent backups)
WAN : 4Mbps Upload, 2Mbps Download (Minecraft can barely, but still run on a 1Mbps Upload connection.)
Please note that Windows OSes uses more resource than UNIX-based OSes, be sure to allocate resources accordingly.
Recommended requirements: (18-22 Players)
CPU : Intel Nehalem-Based CPUs or AMD K10-Based CPUs and better
RAM : 2GB
HDD : 15GB empty space
WAN : 16Mbps Upload, 8Mbps Download
5 Mbs = +-1600 GB a month, when fully used 24 hours a day. So your looking at around +-800-2500 GB a month with 12 hours of active game time and depending on players/mods.
To bad I have no space on my shell servers atm on those it would average out around 2.5-5 Btc a month. (probably the price you should be looking at to get decent hosting in Europe.)
On servers I do have available I can give you these options:
4.00 BtcHD: 40 GB
Trafic: 500 GB
Cpu: 2 * 2.3 Ghz
Memory: 2048 MB
5.00 BtcHD: 40 GB
Trafic: 500 GB
Cpu: 2 * 2.3 Ghz
Memory: 4096 MB
5.50 BtcHD: 80 GB
Trafic: 1000 GB
Cpu: 2 * 2.3 Ghz
Memory: 4096MB
OS: CentOS 6, Debian 6, Ubuntu 12.04 LTS
Hosted in Netherlands (Amsterdam), not flexible on traffic or HD space.
Price may change depending on exchange rate.
I can guarantee a minimum of 10 or 15 Mbs of bandwidth on the vps.
https://bitcointalk.org/index.php?topic=127623.0 Btc is wip but network and clusters have been up for 6+ years so