Conversion from Windows 2003 Server 32bit to 64bit

I am not a terribly advanced computer guru, but I am in charge of our small company's infrastructure.  I just recently bought new servers to upgrade our entire infrastructure (HP Proliant DL380 G5).  As I understand these have Xeon chips and therefore should be able to run 64 bit software.  We have SQL 2005 64bit and Exchange 2007 also 64 bit.  However, we only have Windows 2003 Server standard (32 bit) installed.  Microsoft's website says that I can exchange my 32 for 64 at any time using a media kit.  Although I think this is only for volume licensing (which I'm not sure if we have).  If we don't, do I simply have to buy the 64 bit version of Microsoft 2003 server?  Is there an upgrade version?  Is it a simple procedure?  Or does it require a lot of prep and manipulation?  If we do have the volume licensing (How do I find out?), where can I find this media kit? And what is the procedure to convert the OS?  Is it complicated?  Has anyone had any difficulty?

These are fresh servers.  I am setting up a new domain, so that everything is clean and new.  If it would be better to install the server 2003 x64 as a fresh install, I can still do that.