I'm just about to install EX2010 Enterprise (twin cas, twin Dag). I have a 300 user Ex2007 live server. I presume it won't mess this up! Thanks C

No it won't, just keep in mind that Windows 2008 is Enterprise Edition.Gulab | MCTS-MCITP Messaging: 2010 | MCTS-MCITP Messaging: 2007 | MCC 2011 | Skype: Gulab.Mallah

install 2010 SP1 directly dont go for RTM and then apply SP1

In addition with Gulab !!! Windows 2008 R2 ENT for DAG nodes Windows 2008 R2 STD for CAS/HUB node (You can configure WNLB in STD edition also) Keep witness share into any of HUB and use binary of Exchange 2010 SP1 You can post question if you have any problm further.Anil

Resources: Exchange Server Deployment Assistant Exchange 2007 - Planning Roadmap for Upgrade and Coexistence Planning for High Availability and Site ResiliencePlease remember to click Mark as Answer on the post that helps you, and to click Unmark as Answer if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.