The sensor size is more important them megapixels. For example you can have a camera phone that has 14 megapixels, but it would not take as good a photo as a digital SLR with 8 megapixels. Any Digital SLR over 6 to 8 megapixels will give you very good 8x12 inch prints. Possibly even larger.
We had a grand debate at collage over megapixels vs sensor size, guess what. The sensor size won. Now you can get digital SLR's with full frame sensors as well, equivalent to a 35mm film negative size. New technoligy is always comming out. So if you combine the two, a full size sensor and a large megapixal count, this should be the winning combination for you.
Be it a D700, a D3 or D3s, going up to the D3x in the Nikon range, possibly others depending on your budget, some others here may know other models, as I said they are always changing, upgrading and so on.
Depends on your budget as well.
Happy pixel hunting.