Need assistance or advice on recommended Exchange 2007 architecture for upgrade

We currently have a single Windows 2003 server install for Exchange 2003.  We use a separate ISA 2004 box on the perimeter for the SMTP host.  I need some assistance or advice/recommendation on which direction to go for our upgrade to Exchange 2007.  We have approximately 300-400 mailboxes, divided over 3 different mailstores.  We are licensed for the Enterprise Edition.  
Initial reading led me to think I would try the CCR flavor with 2 clustered Server 2008 boxes and continue to use ISA on the perimeter in place of an Edge server role, however I just recently discovered that if using clustering that the clustered boxes can only have the mailbox role installed, therefore I would then need at least 1 more server to contain the CAS and HUB roles.  Now I'm looking at 4 servers total involved in messaging and the only redundancy is on the actual database server.  
I need some advice here.  If I don't cluster the mailbox server and install the mailbox, cas and hub roles all on the same box what type of fail safe measure can I implement?  Can I still get some redundancy out of this setup?  There is just so much literature out there for Ex2007 I'm getting overwhelmed reading all of these different scenarious.  
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USAF_The_OneConnect With a Mentor Commented:
If you don't go with the CCR in Enterprise you are limited to SCR (Standby Continuous Replication) if you have a second mail server. Or LCR if you just have one mailbox server. Here's the deal, if you go with Enterprise, you will save time and heartache by going with a fully redundant failover/failback scenario with CCR. SCR and LCR aren't designed like a cluster and therefore, failing over is difficult and failing back is equally difficult. It can be done but there's alot of research involved. For example, I'm trying to failover/back using SCR and can only failover but failing back is proving to be quite difficult. So the bottom line and my recommendation is that if you are going to get enterprise licensing, to go with CCR. If you don't, then SCR is in SP1 and that's equally as efficient but not automated and not as easy to implement.

Some things to read up on, Scott Scnoll has a blog out there about Exchange 2007 and some videos about different types of failover/back stuff using SCR and what not. Also get the Exchange 2007 Administrators Guide (Pocket Consultant or the big one will do) and the Exchange Server 2007 Unleashed book. Those will give you alot of configuration setup information and if you do google searches for CCR implementation I guarantee you will find some useful information on Exchange 2007 CCR setup and configuration.
shockeyAuthor Commented:
so, is it correct that if using CCR on (2) clustered Server 2008 servers that I can only install the mailbox server role?  Will this mean that I'll need to have another Server 2008 machine with the CAS and HUB roles.  Doesn't this scenario only give you redudancy on the databases?  
Is there a scenario that would include have the CAS, HT and Mailbox roles all on 1 server and having another standby server with all of these roles as well, and both machines synch to keep the same config and data?
That's a basic configuration. However, it was my understanding that you can install the necessary roles on the two mailservers and the third system was the failover monitor. This provides redundancy for the entire exchange organization. However, I may be wrong in the detailed information. Check this out and see if this helps you out a bit.
shockeyAuthor Commented:
I need someone to confirm whether or not I can have the CAS or HT roles installed on the same box as the Mailbox role if the mailbox setup will be 2 clustered 2008 servers running in CCR?  I have heard that only mailbox servers can be clustered and that if running CCR these other roles need to be on other servers.  I cannot find the documentation to confirm this.

Try this 

Should explain how roles are installed.
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