Dual Opteron Upgrade Question

[soul]

I am building a server and it will eventually be Dual Opteron.  To keep cost 
down I would like to only buy one proc at this time and upgrade to dual proc 
later.  Can I perform an install such as that when I take it down and 
upgrade it with the second proc that it will take advantage of the new 
second proc without having to reinstall.  I really don't want to invest the 
extra money at this time.

If this is possible, which product should I install on the server?

Thanks,
Scott