I like the two tiered system as well. I do have a couple of suggestions for it though.

First, is that what tier you're on is invisible to anyone but admins. I suggest this for a couple of reasons. First is that it attempts to prevent an elitist attitude from being created. The second and IMO more important reason is that if the tiers are visible, it could seriously harm the downloads of those in the new/unpaid tier, because given a choice, automatically people will choose "pro" work over percieved lower quality. Which if number of downloads is going to be the requirement for graduating to pro status, then it really wouldn't be fair to make things harder in this way.

My second suggestion is that the graduating from unpaid to paid should be voluntary after the specified number of downloads is reached, not automatic. This way those who prefer not to be paid, can do so no matter what their downloads are.

I also definately agree on moderating being needed.