http://www.bythom.com/printsizes.htm
At the risk of sounding self-serving, what I find is that most people get much more benefit from solid one-on-one instruction (e.g. workshops) than they do from upgrading their camera to the next number of megapixels. That's because they're not optimizing their picture taking actions, and thus suffer from that downstream. Getting more megapixels actually sometimes works against you, as it pulls out even more shot discipline problems and other issues you haven't mastered but now must deal with in the pixels you captured and wish to reproduce. My advice: if you haven't optimized your shooting with your current camera you can't make the assumption that more megapixels is going to help you get the results you seek.