What is the best way to deal with a large iPhoto library?
The weight of data (in gigs) doesn't really matter with iPhoto. That's just file storage. If you have a large HD with plenty of free space (30%+ and never less than 10%), it really doesn't matter.

iPhoto used to be rated for 250,000 items in the library, but in the most recent update it has the same format as Aperture, and that is effectively unlimited. 

As to how well iPhoto scales: pretty well. I have a 40k plus library that opens in a couple of seconds. I've worked on Libraries in excess of 100k items and while longer to launch (one was almost a minute) it worked fine once open.

You can easily have multiple libraries:

Hold down Option while launching iPhoto, and from the resulting menu select Create Library. Bear in mind that only one can be opened/ searched/ accessed at a time. Again, unless you're getting very large indeed, I would avoid them.

Upgrading to Aperture is simple. Install it. Open the Library. Done.