I am looking to make "the switch" in the next month (cross fingers, arrendale). Well, I too have gigs worth of PSTs. There are critical to me for searching. Here is the rub - only 1 GB of the mail is online in the Exchange server, the other 4-5 gigs is archived, only present in the pst itself.
So, my questions:
1. If I convert this mail, will it still reside in the folders they live in now? e.g. clients, t&e, prospects, inbox, sent, etc - or is it all dumbing to one folder? or some manual step to sync it all up?
I don't have personal experience with the third-party applications that will move your data out of Outlook PSTs and into Entourage.
As I understand it, the third-party solutions will maintain your folder structure. Your best bet is to contact the makers of those third-party solutions to ask this question.
2. I primarily want spotlight indexing this stuff, so does it make sense to move old archive mail to Mail.app and let Entourage do live mail (not-archived)? I would rarely run Mail.app, only when I have a search hit to look into.
Entourage has local folders (that is, folders that are not synced to your Exchange account). If you store your mail in those local folders (in Entourage, they're the ones under "On My Computer"), then you'll be able to search it (both within Entourage, as well as from Spotlight).
3. Can I connect Mail or Entourage to Exchange, yet still import 5 gbs of old mail? Will it try to Sync that old mail to the server (and ultimately fail due to quotas)?
In Entourage, if you have anything in your local folders, that won't be synced to Exchange. Likewise, if you move something from your Exchange folders and into your local folders and delete it from your Exchange folders, it won't reside in your Exchange account any longer.
I do know some folks who have written AppleScripts to automatically move Exchange messages from their Exchange account to Entourage local folders when the message is of a certain age (say, 3 months old). That is, quite honestly, outside of my AppleScripting expertise.

If you're interested in something like that, you might find that the people on the entourage-talk mailing list or in the Entourage forum might be able to help out. More Entourage users hang out in those forums than do here, so those are usually pretty good places to ask Entourage questions.
As an aside, if you're connecting to Exchange 2007 (SP1 RU4 or later), you should use
Entourage for Web Services instead of Entourage 2008. (If your Exchange account is on a beta of Exchange 2010, you must use Entourage for Web Services. Entourage 2008 will not work.) Entourage for Web Services gives you additional Exchange functionality over Entourage 2008, and it's faster too. Entourage for Web Services is included in
Office 2008 Business Edition, and it's a free download for users of Office 2008 Standard Edition and Office 2008 Special Media Edition.
big thanks - mail is the #1 thing making me nervous about the switch. I am a power user of Outlook mail and it looks like Mail and Entourage are both going to fall short of my expectations and this is a biggie to me.
We are bringing Outlook to the Mac in the next version of Office:Mac. It's coming out in time for your holiday shopping pleasure in 2010. We haven't announced a lot about it yet, but there are a few details yet, which are in my blog post
Outlook coming soon to a Mac near you. For you as an Exchange user, we've announced that Outlook will support Information Rights Managed messages. There's more goodness coming, which will get announced on both
my blog and
Mac Mojo, the MacBU team blog.
I would love to hear more about your experiences in moving from Outlook to Entourage, so please feel free to post comments to my blog as you go along.
Regards,
Nadyne.