Hi,
As a general thumb rule we recommend 100GB per Database. You can go upto 5 DB per Storage group, but then again it is upto you. Every storage group add some load onto server so I would recommend that you create one more Database in same SG and move some mailboxes.
It could be based upon Large Mailboxes in EDB and other in one. Else Top Mgmt in one and others in one, etc etc or based upon department/location etc. All upto you.
Also, please see the link I had answered something on Database Mgmt there on how to manage/control sizes, it would be good reading ::: http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/Q_23677038.html
Thanks
Nitin Gupta (gupnit)