A solid speech on one of the definitions related to sin (depravity, fall of man, or original sin) will cross apply to a lot of speeches, as well as the other two definitions. Same goes for justification, atonement, propitiation, redemption, and reconciliation; a good speech on one of those will go a long way. The meaning & significance of the deity of Christ is also really helpful.
In general, I'd recommend starting to build your box with definitions. Good speeches on the definitions help a lot when you're trying to put together a speech in round on general questions or statement analysis.
I have a lot of cards with Bible verses that apply to specific topics (e.g. a set of cards with verses that apply to the fall of man), but that's really it.
That's a great start, and will help you a lot in rounds. I found it really helpful to have some verses and quotes organized by topic; I made sure I had some for each definition, and then as I found useful quotes and verses, I would add them to my cards. That way when I drew the topic, I had a starting place, even if I didn't have a whole outline prepared.