If you use an account with OWA as well as client access you can store recent stuff on the server and archive off older stuff to a local drive so you dont breach storage limits. I get up to 200 emails a day (yeah...a DAY!) at work, so I archive off everything over a month old so I dont breach my 5gb exchange server limit.
My exchange server inbox, sentbox and associated subfolders hover around a total of a thousand emails after deleting all the stuff I don't need, and so load very quickly too. my archives however are 52,392 inbox, 25,345 sentbox and another 10,000 or so in the subfolders. I only access that if I need to...even with 8-cores and a stack of RAM it takes some chewing over by the PC
I also then back up the archive file (.pst file) by copying it to an external drive with the rest of my work backup. It's about 25gb alone
should the computer fail then, the current exchange account just sync's back to outlook and the .pst file imports very easily.
The only Caveat is that you can't access the archive from the OWA....thats exchange server data only.