Can you really scale up with Django...given that you can only use one database? (In the models.py and settings.py)
The database isn't your bottleneck.
Check your browser carefully.
For each page of HTML you're sending (on average) 8 other files, some of which may be quite large. These are your JS, CSS, graphics, etc.
The actual performance bottleneck is the browser requesting those files and accepting the bytes s... l... o... w... l... y...
To scale, then, do this.
Use multiple front-ends balanced with a pure software solution like wackamole. http://www.backhand.org/wackamole/
Use proxy servers like squid to send the "other" files. They're largely static. This is where 7/8ths of the work is done downloading to the client. Don't scrimp on getting these right.
Use multiple, concurrent mod_wsgi/Django to create the -- rare -- piece of dynamic HTML based on DB queries. Be sure that mod_wsgi is in daemon mode so that you can have multiple Django servers available to Apache. Build as many of these as you need. They're all identical, all in parallel, and all shared by Wackamole.
Use a single, fast database like MySQL for the few things that must come from a database. MySQL will make use of multiple cores on it's server, so it will scale reasonably well without you having to do anything other than buy memory. Put this on a separate box, all by itself, dedicated and tuned for just this.
You'll find that this scales nicely. You'll find that the load is shared nicely between squid, apache, the Django daemons and the actual database. You'll also find that each part of the load (from the boring static parts to the interesting database query) happens separately and concurrently.
Finally, buy Schlossnagle's book. http://www.amazon.com/Scalable-Internet-Architectures-Theo-Schlossnagle/dp/067232699X
Read scaling to millions of users is not a database problem, but is fixed with load balancing and caching, etc, see S. Lott above.
Write scaling can indeed be a database problem. "Sharding" and having multiple databases can be one solution, but that's hard with SQL while still retaining the relationality of the database. Popular solutions there are the new types of "nosql" databases. But if you really have those problems, then you need serious expert help, not just answers from dudes Stackoverflow. :)