I'm not sure I understand. Next actions really are a todo list - just better defined and concise than what most people refer to as a todo list.
As for the 3x5's I think they're great. You might consider a wallet that holds 3x5's though and has a place for a small retractable pen like the fisher space pen. I recently invested in this setup and it works like a champ. I only use it for collection though. If you're going to process and organize with them you may want to go the whole route and make your whole system revolve around 3x5's. But where would that put your calendar, phone book, etc.?
I think its best to have all that in the same tool.