1. Gaming support on the Mac is nowhere near as good as on the PC.
2. Not easy. You're really stuck with expensive Apple upgrades. And the cheaper (that term is relative - there's no such thing as a cheap Mac) ones are very limited in upgrade possibilities.
3. I'm not sure. Check out Steam and compare prices.
4. Much more difficult. And there is less of a hacker base out there to help you. Mac users tend (it's a generalization, but there's a lot of truth in it) to be people who want their computers to just work without tweaking.
I have a Mac and some PCs; I would never recommend a Mac for gaming purposes. If the only reason that you have a desktop computer is for gaming then it's a no-brainer. Keep the PC and spend the $1000 or more that you'll save on upgrading it.