Promote a Windows 2008 R2 server as a domain controller

try http://www.brajkovic.info/windows-server-2008/how-to-promote-windows-server-2008-machine-to-domain-controller-step-by-step-guide/

Here's a very handy Microsoft article on it all.

http://technet.microsoft.com/en-us/library/upgrade-domain-controllers-to-windows-server-2008-r2(WS.10).aspx

No nothing major in terms of problems with the 2003 boxes.  If your current 2003 boxes are 32 bit then make sure to use adprep32 from the 2008 R2 media.  If you are running DNS on your current DCs make sure to install DNS on the new box and also make it a GC (both can be checked during dcpromo)

Some things to look out for in terms of virtualizing are time (don't sync with the host let the windows time hiearchy work).

Also don't ever use snapshots to restore the virtual DC

Some good blogs

http://blogs.technet.com/b/askds/archive/2010/06/10/how-to-virtualize-active-directory-domain-controllers-part-1.aspx
http://blogs.technet.com/b/askds/archive/2010/06/15/how-to-virtualize-active-directory-domain-controllers-part-2.aspx
http://blogs.technet.com/b/askds/archive/2009/06/05/dc-s-and-vm-s-avoiding-the-do-over.aspx

Thanks

Mike

is it easy enough to upgrade existing ones or should the neqw 2008 os be installed from scratch

I'm in favor of installing from scratch if you are using virtualization that is even easier.

Thanks

Mike

the majority of our dcs are physical!

I like from scratch for all DCs.

Also remember an upgrade in place for 2008 R2 means the current box has to be 64 bit.

Thanks

Mike

am i right in saying as long as it has a 64bit cpu it should be ok?
are there any problems running 2008 r2 dcs and standard 2008 dcs?
will i only get the benefits if i have the complete infrastructure 2008 dcs, or is it ok to have a mixture?

many thanks