The opening effect should establish exactly what you're trying to demonstrate. For me, if you're going for a cheating demonstration, flashy moves isn't the perfect choice (at the very least, not for the first effect). I would start with something more simple, something that builds the overall theme of the routine.
Myself, when I have done gambling/cheating routines of this sort, I begin with a simple conversation about playing cards, greed, and the desire to win. This is all done casually as a "fair" game of poker is dealt out. The cards are shown to be random hands, some good, some awful. Pointing this out, you talk about how greed and desire for money, for an edge - brought out the worst in some folks. They didn't want to rely on luck, they wanted to tip the odds in their favor.
As you're doing this, you've gathered up and shuffled the cards, dealing out another multiple hand poker game. At the conclusion, you turn over your cards, displaying the four aces, and say something along the lines of...
"And from that desire... the cheater emerged".
You can then go into any style routine you like. I used to use Dai Vernon's poker deal as my opener, but I eventually trimmed it down to a more simplified and casual presentation under the premise of a simple game of cards. The Vernon Poker deal is still, in my humble opinion, one of the best gambling demonstrations going.
So, to echo some of the others here - go for something more simple to start. It can still be powerful, but I like to set the tone with my first effect, and build upon that.
Michael - I do believe the "shotgun aces" in question is something like this:
http://youtube.com/watch?v=75STI8_SwEc
I also agree with Michael about keeping it to one deck and the drawbacks of over proving. I see the logic in introducing a new deck as playing up how fair everything is, but I would focus more on convincingly "losing" the aces in the deck and working from one deck, rather than breaking your flow and working in a new deck.
Remember, there's an underlying theme running through this from the start. You are TELLING THEM this is a demonstration of card cheating. They already know something is up. It's not the deck that is suspect, it's you. Your demonstration, by the very nature of what you're talking about, somewhat implies that you are working from a normal deck of cards. It's not the cards that are tricky - it's you.
This is the difference between a purely magical presentation and a gambling demonstration. For me, the necessity to prove your deck as "above board" has never been a problem. At least, that's how it was when I worked these kinds of routines. It offers a sense of freedom (in that people won't bother you with "let me see those cards"). However, your technique needs to be flawless...