Well, I'm a big fan of not operating outside of your comfort zone so I'm apt to say go with your gut and progress when you're ready.
I'm going to bet that the jump from 20 to 40-45 is going to be less of a shock than the jump from quiet residential streets to trafficked streets.
In that vein, your idea of hopping on the street when it has very little traffic sounds like a good one. That way you are separating out the two new experiences and can (hopefully) become acclimated to each individually before having to tackle them together.
I think you'll find that the increase in speed isn't that big of a deal - especially if you don't concentrate on it. Just accelerate, don't stare at the speedometer, and you'll probably find that you're very comfortable going faster on a road that is built for the higher speed.
And, if you do encounter any traffic... don't let them stress you. I recall stalling a lot at lights when I first started riding and people behind me would (understandably) get frustrated. But, just because the people behind you are impatient, don't try to rush yourself or do something you're not ready for just to make them happy. They can suck it up.
Hope this helps... ?
[b][i]"Good girls go to heaven. Bad ones go to hell. And girls on fast bikes go anywhere they want." [/i]
2009 Triumph Sprint ST - daily commuter
2006 Triumph Daytona 675 - track bike
1999 Suzuki SV650 - track training bike[/b]