Generally, megapixels in a camera has very little to do with how large you can print photos. There are so many other things like colour noise (grain), JPG quality, blur, etc that camera salesmen don't touch on.
I have the Nikon D40, and it is great. Even though it has "just" 6mp, the image quality is simply superb. To cut a long story short, the D40's 6mp photos will print stunning A4 prints and great A3 prints. It will even do slightly larger if pushed, but only beyond A3 will you notice a real difference.
Any entry-level DSLR will do for pretty big prints, but I, personally, love the '40. Try out some alternatives at your local camera shop.
And if you do go with the D40, don't be drawn in to the "AF-D lenses won't work on the D40" discussion. Basically,
1) The lenses will work, but you won't get Autofocus.
2) There are alternatives for basically all of Nikon's lenses (apart from the 50mm F1.8) that you'll need.
Good luck!!!
P.S. 300 DPI (dots per inch) is the standard for pro-quality prints, but you can get away with far, FAR less than that.