With later client releases I think you can expect about 10 to 15 kB/s of bandwidth for each player.
Download would be more used than upload, (server is some kind of player "repeater"), processor wouldn't be a trouble for a server under the 600-800 range (judging from genesis experience) and memory could rise in usage compared to a fresh launched shard (about 600-800 MB of memory when launched on Windows, about 3-6GB on linux...)
I still haven't found what makes Mono Memory usage this high, it's really about 5 times bigger than using windows !
For a production server use I would recommend this config under Linux :
- Quad Core Processor (or equivalent like 2C/4T processors), ranging from Atom (maybe 300+ player limit) to Xeon (maybe 800+ player limit due to DOL weakness)
- 16 GB Memory (if the database is hosted on the same server), 8GB can be enough for testing or if you take care of not running too much services from the server (reduced MySQL memory Usage, no Web server...)
- Any RAID software that can handle more than one storage device, Databases have better performances when they can queue more parallel I/O commands !
The only way to have the best of each world would be to run a Windows Virtual Machine under a Linux Hypervisor, only handling the DOLServer Binary, then connecting through TCP/IP to the host which can run Database and other services !
A Windows VM with 3-4 GB should be plenty enough for DOL which doesn't use a lot of I/O, you can then run a fully working official .NET Runtime to host DOL and have enough hardware on a 8GB server !