As a school, we also are being mandated to have a email archive in place. I will be going with the Arcmail Defender product. It has great reviews, doesn't require another server for data store; when I tried out GFI a few years back it was pretty simple, but it did house it's data on a SQL server; and it's a very mature product. I also looked briefly at Barracuda Networks new mail archiver appliance. This was a pretty good first attempt at this and I feel in another 3-5 years, they will have a very rounded proven appliance, but not right now. Arcmail has been doing this for many years now and they have a very mature, solid product that is top notch.
Run through there web demo and if it looks good, you can even request a eval unit to test on your network...can't beat that. Hope that helps.