My shot that we had enlarged was from a Canon 5D MarkII, 21 megapixel camera and I have them printed at 300dpi
Sorry Pat, that should have read ppi not dpi, my bad. dpi, ppi, it's all so confusing.
Anyway 21 megapixels, that gives you what, about 5616 pixels on the long axis and 3744 on the short. So that works out to 156 ppi on a 24x36" print. Personally I find below 180 ppi you start to see the edges of the pixels themselves which gives you those little jagged edges that I personally don't like seeing. IMO, to make a half decent 24 x 36" print, one that could pass a scrutinized inspection, we need at least 28 megapixels to get us to/past that 180 ppi mark. For a superb print at that size, say one with at least 280+ ppi, we need almost 68 megapixels,... and I guess that would be why medium format is king for large print commercial work.