Continual Mail crashes - possible mailbox is too big?

Question

Mail crashes for me daily, if not multiple times a day. This morning I couldn't get it to load new mail without crashing and giving the error below....
One thing to note is that I have had this Macbook for 3 years and use it 99% for Mail, when I purchased it I imported my old Mac Mailboxes to keep the data from old mail as well. The Mail folder in Library is 8.2GB. If a possible solution is cleaning out the folders - could someone suggest a good way to archive everything so that I can still search for mail if need be? I use that option a lot.
I have tried removing the plist and then putting it back, there is also a 2nd plist (com.apple.mail.plist.0THHeZo) which I'm not sure about.
I believed it to be an address book issue for a while since I have 3,000+ contacts which I need, but even after deleting that plist and re-syncing with MobileMe, it seems the application itself is the problem.
Error:
Process: Mail [1100]
Path: /Applications/Mail.app/Contents/MacOS/Mail
Identifier: com.apple.mail
Version: 4.4 (1082)
Build Info: Mail-10820000~1
Code Type: X86-64 (Native)
Parent Process: launchd [818]
Date/Time: 2010-12-27 12:29:50.464 -0800
OS Version: Mac OS X 10.6.5 (10H574)
Report Version: 6
Interval Since Last Report: 46888 sec
Crashes Since Last Report: 4
Per-App Interval Since Last Report: 22925 sec
Per-App Crashes Since Last Report: 4
Anonymous UUID: 40475077-8D94-4E32-8798-3C162D72E77F
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Crashed Thread: 27 Dispatch queue: com.apple.root.default-priority

Answer 1

They're all under 100mb except for my Sent Mail folder, where I archive all my sent emails, is 1.45GB but split up in 3 different folders.
I never knew that, though, so I will keep tabs on it.
Is it anything like Outlook where I should just archive more items into separate personal folders to spread things out? Or do I need to move folders off the servers if that is the issue?
Either way, Mail is beyond sluggish. Only one of those multiple email accounts gets a lot of daily email so it's not thousands of messages daily. The Exchange account only works if I am logged in from the office - could it be trying to access that constantly that is killing the memory?