The Antec 300 is a nice fairly roomy case, it has more than enough space for a 4850, the NZXT is a bit larger and both will have good cooling, so it really comes down to personal preference, and if you are going to use that rebate. If you arent going to use the rebate then i would go for the antec since its significantly cheaper, if you are going to use the rebate then get which ever one you think looks cooler.
I bought the 550w Fatal1ty when it was on sale a few days ago. Did a combo with a dvdrw drive and got both for $60. It has pretty decent ratings. I really wanted a corsair but i'm cutting cost by buying stuff on sale. Starting to wonder if this was a mistake. I heard bad things about ocz rebates too.