My company is finally listening to me and looking to implement some forums for tech support and to get rid of our old mailing lists. It'll start small, but we have like 500,000 users, so could grow pretty large over time.

The task of recommending the right software falls to me. We'll be administering what we can, buy prefer not to host it on our servers.

I do like the vBulletin they're using here. I was a mod on another board (now defunct) that used phpBB. I liked that and it was pretty configurable, but I'm worried about scalability. Once we had a few hundred users there, it seemed to get a bit bogged down. The current opinion of my boss is anything using MySQL would be fine with him.

So, what's the best forum software? Commercial is okay, after all this is for a software company.

And I'm not in charge of finding the host, but recommendations there too would be nice.