Windows Server Virtualization

can any one please be very specific about how much storage is required for virtulized server and how can affect if hard drive storage is not enough on PDC?
what kind of system symtoms would I get?

Explain:
I am not sure this is helpful but here it goes, assuming small business with 5 Servers 51 users on a Full T1

1 PDC Primary Domain Controller
SATA RAID 1 (800MB FREE)

1 2007 Exchange Mail Server
SATA RAID 1 (80GB FREE)

1 MS-SQL 2008 Server
SAM SCSI RAID 5 (500GB FREE)

1 MS Report Server
SATA RAID 1 (250GB FREE)

1 IIS 6.0 Server
SATA RAID 1(180GB FREE)
5 Domains

All Virtualized.
I hope that the info provided above are helpful.

TIA
NC