The problem with multiple libraries is that they are inconvenient. Only one can be open and searched at a time, so if you're looking for that photo from your trip to NY, well was that in 95? 96? Maybe it was 94... And as an Aperture Library is pretty much unlimitied in size, I don't see a compelling argument for a library-per-year approach.
Having two libraries - one on the internal and one on the external - seems like a lot more convenient, but I would do that slightly differently.
On the External I would have one complete library. This makes backing up very easy. Back that up and you have everything backed up. From that I would export as a library the material you want to take on the road with you. If you import then ideally import to the one main library, of if not, then you can move material from the internal to the external library using the export as Library/ import library function. So, at all times the internal library is a sub-set of the main one.
I'm wary of Dropbox and other cloud solutions for backing up an entire Library. Why? These are very large databases. I've seen people complain that restoring a library can take days simply because these guys shape their traffic and you're downloading massive amounts of data.
Further, many cloud services store the files on inappropriate disk formats, and this can lead to damage to the Library.
So, my suggestion would be as follows:
One full size Library on an external
That backed up to another external.
Ideally, backed up to a second external - this lives off-site at your workplace, a relatives house, even in your car.
On your internal a subset of your main library.
As for backing up Photos to the cloud, that's quite simple. Services like Flickr and SmugMug - and there are many others - are there and you can upload all your photos, they remain accessible from any computer anywhere in the world.