Thank you both for your replies. I have between now and summer and then 4 weeks within summer to carry this out. I have the new Vanilla servers in the building with server 2008 pre-installed and partitioned ready for the change over.
What I ideally need now is some advice how to go about this in the correct order to get it right first time. I am fully aware and appreciate this is going to be no small task. Luckily our staff laptops don't join the domain so in that respect thats 80 less machines to worry about. In total I have around 900 users to move and 360 machines to re-image.
If I join the new servers to the network, promote them all to DC's put active directory on all 3, DNS on all 3 and leave DHCP off, should I then be able to start setting up GPO's and RIS to roll out Windows 7 via PXE boot.
Any help as I say is greatly appreciated.