Wow. If all you are doing is making posters - ANY camera on the market can do what you are looking for it to do. Image quality is largely meaningless in this day and age. It's the lens that you attach to your camera that is going to matter.
As for your combo here is what I would suggest: Stick with Canon or Nikon. If you go for the Canon, I'm sure you can pick up a refurbed 50D (or maybe even new) for about 1000 bucks or so, which would leave you with nice wiggle room to splurge on lenses (and some lighting equipment, depending on what kind of posters you are wanting).
For Nikon, I would suggest a D90 for 900 bucks refurbished from
Adorama, and similarly coupling it with appreciable lenses (although, even at a premium, I would go with the more "pro" bodies Nikon D300).
And no, you are not spending money on the video feature. That is going to be in most cameras moving forward. You are paying for a full frame 21MP camera. As such, the 5D might be a better option for you as it has more detail grabbing capability, and if you are "sweating" upscaling your images (which you are going to have to do regardless), then this might be an option for you. However it IS a hefty cost and going to eat up much of your budget and leave you with not a whole lot of wiggle room for accessories.