I'd go for the D5100 in your position. Its got a better sensor and features than the D3100 while being cheaper than the D7000 but comparable in image quality. I think you'll also appreciate the extra pixels if you're trying to print that large. Also, when you're just beginning, you'll find your money is best spent on lenses rather than higher-end bodies ... particularly when you don't know how to use the advanced features that set the D7000 and D5100 apart yet.
I'd recommend making a list of what lenses you actually want to use. My guess is the kit lens won't be on it, and if that's the case, you're better off buying the body only.
At the college where I work, we're setting up bags for student photographers to grab when they are on assignment. Our list might be rather different from yours, since we're mostly concerned with events, athletics, and some portraiture, but it might give you an idea of what you may want. Here's what we decided on for each bag:
Nikkor 50 mm f/1.8 prime
Nikkor 85 mm f/1.8 prime
Sigma 17-50 mm f/2.8 zoom
Sigma 70-200 mm f/2.8 zoom
We figured with those four lenses, the students could handle just about any assignment. We are also on a limited budget, which is why we went with the Sigma lenses. I've had really good experiences with Sigma, and have found them to be on par with the more expensive Nikkor offerings in just about every way except weather sealing. My experience with Tamron hasn't been as good, mostly because of auto focus issues. The videographers in our office particularly love the Sigma 17-50, which is a much faster lens than the 18-55 kit, giving it more flexibility in tricky lighting situations.
I hope my ramblings help. Photography has a steep learning curve, and I'm certainly no expert yet, but it's been a rewarding journey so far.