Minecraft – How much RAM does a Minecraft server need for a Spleef arena

Tip: If you want to spare more memory, you may set the -Xms parameter even lower, say:

java -Xms32M -Xmx512M -jar minecraft_server.jar nogui

The parameter controls how much memory is reserved on startup. Your server will start with 32MB RAM and whenever it needs more memory it will allocate some until it reaches the maximum of 512M. However, this will result in a little slowdown whenever the allocation is done.

Source: http://www.minecraftwiki.net/wiki/Tutorials/Setting_up_a_server#Less_than_1GB_free_RAM

Also you can check this website : http://www.canihostaminecraftserver.com/

With this you can have a small idea on how many players you can handle :D

On a purely vanilla server, you can't. There are no built-in tools or commands to move the spawn point. However, without necessarily using mods like Bukkit, you can still use external tools to modify the level.dat file to change the spawn point.

You can use a program like NBTEdit or NBTExplorer (or NEINedit if on a Mac) to open your level.dat file inside your world folder. Then change the values for SpawnX, SpawnY, and SpawnZ to the coordinates you'd like for your new spawn.

Note that moving the spawn will also move the protected block radius, so keep that in mind.