Server overloaded? need help-.-

heya,
I've a problem with my server, it says "[WARNING] Can't keep up! Did the system time change, or is the server overloaded?"
Screenshot
I've massive laggs, what can I do?

My Server:
-Debian5 x64
-4GhZ CPU
-8GB Ram
100GB HDD

my Craftbukkit: #1337
Plugins:

Code:

-bChat
-BCMusic
-BlockHat
-BookWorm
-bPermissions
-BuyCraft
-GlowstoneLanterns
-godPowers
-Healthbar
-Inception
-Lockette
-Logblock
-Minequery
-motd
-MyHome
-MyWarp
-noEnderman
-SimpleSkylands
-Spire
-Spout
-SpoutTrade
-SpoutMOTD
-Stargate
-Tables
-UltrItems
-Vault
-Votifier
-WorldEdit
-WorldGuard
-YouControl

I've started the Server with this command:

Code:

 java -d64 -Xincgc -Xmx6G -jar cb3.jar nogui

the max heap size go up to 768, why?

Code:

 [INFO] New max size: 768

Can anyone help me?