What gaming PC should I buy (or build)?

This might already be a question asked by several people, but there are too many posts to filter through to find out.

I am a graphic designer who formerly owned a custom built gaming PC before I could finally afford to purchase a Mac (I now have 2; 3 if I count my work computer). I'm somewhat ignorant these days when it comes to this stuff, forgive me. I mainly game on consoles (360/ps3), but have been yearning to get back into PC gaming. So, basically, I am asking two things:

  1. Is there a good source for great games (new and old) to play on the Mac? I know there is Steam, but I haven't found much better than Half-Life 2, Team Fortress 2 and Portal 2.
  2. If not, what gaming PC should I buy? Or should I build one? Just looking for recommendations/advice.

Note: I am not looking at a high-end machine. Probably more mid-range? Nothing more than, say, $1,000?

Thanks random people! :)