Advice on Fastest NMAP Scan

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