Before kicking off the actual implementation stage, verify that the required third party products in your company (e.g. Antivirus, Backup, etc.) support Exchange 2010 and Windows 2008 R2. Remeber you may well be able to cut some licence cosots with new functionality now available in Exchange 2010 - backupless exchange on JBOD, online archive etc, etc.
Although you can install Exchange 2010 on Windows 2008 x64, I strongly recommend using Windows 2008 R2 as it provides a higher performance and greater flexabilty. A really strong "better together" story.
To use all Exchange 2010 features, you may need to upgrade the clients to use Outlook 2010, or move to an Outlook Web App model. This is today an obvious issue and potentially a project blocker if you need online archive access etc. Exchange 2010Sp1 is almost here and with it a good number of new feature enhancements and funtionality porting.
Exchange 2010 works in RAM to complete as much of its workload as possible, we have dropped our IOPs to a level where we can support JBOD!. I really recommend adding sufficent RAM to the server by estimating the future overhead on the server. Make sure you do your calculations with real world data gathered from the current environment. Make sure you use the latest version of the Storage Calculator to get the best numbers.
Microsoft is doing a great job and is completely focused on fixing the identifed issues within Exchange 2010. Please ensure you upgrade the environment to the latest patch roll up or service pack before starting the move of mailboxes to the new Exchange 2010 environemnt
Use Windows 2008 R2 Enterprise edition, so you can expand your Exchange 2010 DAG beyond 5 databases as your organisation grows.
Using Microsoft Best Practice analyzer on a schyeduled basis to help you find any possible new or growing issues in your environment.
Microsoft has now released the tool “Exchange Pre-Deployment Analyzer” that can help you to map the current Exchange 2003/2007 environement. This is a very good tool to help identify issues and showstoppers prior to installing the first Exchange 2010 server. Use it! You can download this tool from this link.