1. Your PSU has more than enough SATA power connectors. You might not have enough SATA cables that come with the motherboard (you'll need to check the packaging details), if not, just buy some more. You won't run out of SATA slots for all the input devices you've listed (there's more than enough connections even for future upgrades and additions).
Pretty much regardless of what the benchmarks say. In terms of real life gaming wise, the HD4850 will last you about as long as the 4870 and the GTX260. What I need to know is what resolution you plan to play games at? Depending on your resolution, I'd choose differently.
And yes, the P5Q-Pro is reliable (if you get a DOA board, it's luck, it happens).
MM, looks fine, Maybe you want to just get 2 Burner's instead? It's roughly the same money (doesn't really matter). Maybe add in the Xigmatek cooler that I mentioned earlier if you ever want to try overclocking a bit (it's easier and safer than you think). Check out the overclocking section of the forums.