A foreign friends for help,on the server configuration

First of all, to everyone ask sound good afternoon.
I come from China, a friend
We have a lot of minecraft bukkit like friends
My English is not enough to communicate, I'm sorry, can only use the software to translate.
I established a server, but can't keep stable, I want to let more people to join my server, I need to replace the configuration
Configuration is as follows:
CPU: Intel Xeon E5620 2.40 GHz (133 MHz x 18.0) 4 core
Memory: DDR3 1333 8 G X2 ECC as CPU only support to 1066
Asked how many players can play together