Planning an Exchange 2007 upgrade strategy from Exchange 2000

Hello,
I'm in the process of planning for an Exchange 2007 transition from an Exchange 2000 environment.  I currently have around 300 mailboxes, so it's not a very large company.  I'm currently running a two-node Exchange 2000 cluster (using your basic Microsoft Clustering service).   I have an SMTP server sitting out on my DMZ for incoming email (outgoing email just gets sent directly to the Internet).  I also have an OWA server sitting out on the DMZ as well.  (I personally would not put an OWA server on a DMZ, but this was inherited).

What sort of strategy would be recommended?  From what I read and have heard in training classes, is a big push for Microsoft ISA on a DMZ that then publishes to a Client Access server for OWA access.  In our smaller environment, that seems to be a bit overkill.  Another option is to put a CAS on the DMZ (to act similar to our current OWA).  But again, the whole "member server in the DMZ" bothers me.  Any recommendations?

And finally, the difference between SCR and CCR, to me, is minimal.  I understand that CCR takes away the whole shared data single point of failure theory, but I'm thinking more ease of setup and similar to our current environment.  Any strong recommendations for CCR over SCR?  What additional costs are involved with CCR?  is it just the witness server?

Any thoughts or recommendations would be greatly appreciated.

Thanks!