Advice on Fastest NMAP Scan

["Mogren, Jack L." <mogren () mayo edu>]

Hi,
  I want to use NMAP to perform a network inventory on our network.  We have over 75,000 networked devices.  The 
address ranges include one class B, several class Cs, and the entire private address space.  That's a lot of addresses 
to scan and I expect it to take a long time.  I'm looking for advice on what options one might use to create the 
fastest NMAP scan.  I'm only interested in finding our each devices OS and open services.  Here's what I've come up 
with so far.

nmap -O -T4 -PE -F --osscan_limit -oX /home/security/test.xml -iL /home/security/ip_addresses.txt

  Any comments or suggestions?

Also,  has anyone had luck importing a NMAP-generated XML file into MS Access?  When I do so, it creates the tables, 
but the fields are blank.  No data.

Thanks,

Jack Mogren
Mayo Clinic
Foundation Information Security Office