Either the Antec 1200 or HAF 932 are good cases. That said, I wouldn't buy either if you have an $800 budget. You case is important as far as cooling is concerned but the guts are more important from a gaming performance standpoint. I would recommend checking out tecmo34 Builds by Usage
. He has more or less done the leg work for you and all the parts he selects for the builds are quality parts.
Regarding the video cards, ATI makes a nice card. I personally use a 5870 , but that will blow 1/2 your budget. A 5850 will run you about $300. If you have a particular game you want to play, I suggest googling "<game name> benchmark" and you will likely see how different cards run the game(s).