We have had some email outages in the last few months and my boss is asking is it worth going with one of those email redundancy/filtering sites. The thing is, we have a backup server on a remote location, so I am pretty sure we can set something up with what we have already.

At the moment I have the top MX record pointing to my in-house server and the next entry for our dedicated (off-site) server in a data center. I was thinking that instead, we should have the first record pointing to the off-site server, which would save all emails and then forward them on to the in-house server. That way we would have backups etc. In the event of that server going down the in-house one would take over. The off-site server is linux and the in-house is SBS.

Is this possible, and are there any resources out there to help me?

It's not exactly clear how you're doing backups, perhaps copying messages via IMAP, but generally you can specify your MX's records priority, here's an example with Google :
Which means : the name server ns1.google.com knows that the domain 'gmail.com' has few MX records, and the highest priority is email server with name "gmail-smtp-in.l.google.com". If it fails, for some reason, the next upper number server will take care of the mail delivery.
Actually, if you ping each of the top 3 server, you'll notice that they are in a totally different network segments, which is a good practice. Imaging an erroneous router rule, for denying access to 10.101.1.0/24 Just an example