Before committing the whole hardware, I recommend doing a small-scale test first, see if the server software supports all of your needs (if you want survival or redstone, you're out of luck - those are quite under-developed currently).
Hardware-wise, I think the CPU is not as much a bottleneck as the RAM. Depends mostly on the players' viewdistance, but I'd approximate one player per 128 MiB of RAM to be safe (8 players per gigabyte). I believe the CPU alone can support hundreds of players without much strain, unless all of them go exploring a single world each one in a different direction, then it'd lag terribly
EDIT: Those are my slightly-educated guesses, please don't take them literally. I have never actually run the server with that many players.
Hardware-wise, I think the CPU is not as much a bottleneck as the RAM. Depends mostly on the players' viewdistance, but I'd approximate one player per 128 MiB of RAM to be safe (8 players per gigabyte). I believe the CPU alone can support hundreds of players without much strain, unless all of them go exploring a single world each one in a different direction, then it'd lag terribly
EDIT: Those are my slightly-educated guesses, please don't take them literally. I have never actually run the server with that many players.