With the numbers you are quoting on the backups, with the sizes of the databases, looks as though don't go through that much in changes. ~10GB of churn in a year is nothing for Sharepoint. I'm thinking you have a few numbers off. Ive seen it in clusters of 20-30 front ends with two massive database servers behind them churn 30GB a day.
Easiest way to get some scalability is to make sure your database server has enough horsepower. With that much data, a two proc/32GB machine should be sufficient. Get it into a Windows Failover Cluster, and you should have plenty of room to run.
The front ends are much easier to deal with. Load balance them using whatever and have them join the farm. Adding a web node is trivial in Sharepoint.
The rest is just knowing what you plan on doing in Sharepoint. If you are just presenting a website and using it for a CMS, nothing to it. If you are using any kind of analyitics to parse data, then there is more towards queries and database IOPS. If you are using it as a data repo, then giant database to hold the blobs would be important.
What are you running now? What seems to be bottlenecked?