Once again I've gotta say it's subjective. What do you consider acceptable? Do you want posters that look good when you are standing right in front of them or from across the room? There is a bit of difference in those two situations. Both require a good lens and good lighting but you can get away with a little less camera to get results that look good from across the room.
I agree with what dripdaze said. There are a lot of factors and if you do not know a lot about cameras it can be difficult to find the best "bang for the buck". A lot of people will say that if you want to be able to take images that will be good for poster prints than just get the highest megapixel you can afford. That is simply just false. Pixel count is important but perhaps even more so is the quality of the camera sensor. And most beginners won't even know what to look for as advertising or info on a camera box will make even the worst quality camera sound like it is something great.
My advice.. ask lots of questions and recommendations. Go to Flickr and look at lots of photos (as most tell you what type of setup was used) or ask others on here for pictures that they have taken with their own camera. The proof is in the pudding. 8^)