At the end of the day it all depends on how large you are going to print. If you are printing 20 x 30 at the largest, 8 MP is more than enough....given it is 8 MP on a good sensor. For example, 8 MP from a 1D2 vs 8 MP from a Rebel XT. There's no comparison, the 1D2 is head and shoulders above the Rebel XT....despite the fact they both have the same number of MP.
Now, if you lack having a long enough lens to get close enough and you are constantly having to crop in to get the best composition, then maybe you need more MP. For me it is simple, if I can't get the shot, I can't get the shot. I see a lot of people shooting subject's that are just too far away and then getting upset because they can't get a sharp image from their 100% crop. LOL Buy a longer lens!
I have seen images printed up to 40 inches from 8 MP cameras. So, unless you have a high demand for 60 inch prints, I personally don't think you need more than 10 to 15 MP. But, that's my opinion. I would rather buy a good body in that range and then spend my money on glass.