Tier 1 is anything from 5-10 dollars, you get the blue italicized name and access to the forum with extra styles to test (nothing special, trust me).
Tier 2 is 10-15 dollars, you get all the profile privileges, extra space in your PM box, and you can make a blog.
Tier 3 is 15-20 dollars, you get a larger avatar (150x150) and access to the VIP Forum.
Tier 4 is 20-25 dollars and you get even more PM space.
Tier 5 is 25 dollars or more and you get a Pokecommunity email.
There is a tier 6 but it's not mentioned on the donations page and there's nothing special about it, I'm not sure how much you need to donate for it haha.
As far as becoming a mod, the FAQ says it better than I do: