Kev, I am not sure how "big" you really mean, if your "wide format" is talking about A2, and you are new to printing, my experience may be of use. I bought my first serious printer, the Epson 3800 an entry to wide format, last Christmas. When I browsed through the library looking for something worth printing big, (for me A3+ was pretty big) I found the reasons I dismiss a candidate were usually off focus, imperfect exposure and shaken image, but not resolution not high enough. One of my best A3+ print was taken with my old D70, 6Mp image. With the 12Mp of my D300, I am comfortable to go up to A2 (limit of the 3800), provided everything is right. It is not possible to use tripod, mirror all the time, but they really more critical then camera pixel count in real world.
My suggestion is you go get a good printer now and start printing. Print something big, look at the print, look at the image zoom to 200% or more, you will be able to tell you need more pixel or not. Everything being equal, higher pixel is better, but in real world, a camera is unlikely the most effective step to take. My experience is there is a lot money to spend after you get a printer, papers, ink, a faster computer, second monitor, software, hardware for colour management, reference books, sharper/ faster lens may be.