Never use any flashlights... never! First of all it's mostly forbidden even for professional/accredited photographers and secondly you'll annoy both the band and the other photographers with your flash. You'll just have to rely on available light. I know that is really hard but at least you'll end up having a big challenge at smaller venues and clubs.
For the big stadium concerts there's no need to worry anyway, even my grandma can take pics there with the amount of light you have available there.
What size were you thinking of printing anyway? I've done an exposition on my concert photography recently and had a few photo's printed at 50x70cm (18x24") and even those made with my 10MP D200 looked absolutely great with lots of detail. No problem viewing those even closer by than you are supposed to. There really si no need for more than 12MP unless you are going to crop a lot, and that shouldn't be needed if you frame well.
As for the required ISO, 3200 is like feeling happy, I have to use 6400 ISO most of the time to get my minimum shutterspeed of 1/100s, eventhough there are plenty of concerts where I get no more than 1/20s at 6400 ISO @ 2.8. Yes concerts can be that challenging, maybe the hardest form of photography. But hey you don't have any control over light and subject so at least you can blame others for bad shots ;)
No upgrading for several years, nice idea but depending on your needs and amount of pics you take at a concert combined with the amount of concerts you do at a year, you can start saving to get a new camera every 2 years anyway. Although my D200s are still working fine well after 250,000 shots they will end up dead some time just like my D70 did after 250,000 shots (2x replaced the seq motor, now AF motor is broken). Now I know I make a rediculous amount of photos a year with my cameras but still, you'll have to go for a life span of about 3 years at max. Little over 50 bucks a month to set aside for a new camera.
You'll however also need quite a few lenses to cover your work. Even at small venues your 24-70 won't get you anywhere in many cases. For half frames and close-up you'll need to go on stage or be right at the front, though you'll end up seeing nasty nose holes and dubble chins! My kit for any concert consist of 14-24, 24-70, 70-200, 50, 85. You never know when you need what, all depending on the act and the show you'll see on stage.
So bottom line, don't wait for more MPs, you won't really need them. Waiting will make sure you miss out on some great concerts. And I surely hope you are in the USA and not in Europe, no way yo can sell any pics here. Trust me, I am amongst the top concert photographers of the Netherlands and know they won't buy, they'll just go for some lesser shots they can get for free, so many people with cameras always a pic they can use. I'll just get my kicks out of having my name on cd/dvd covers and so, after all it's just a hobby for me.