The case I had originally seems to have adequate air cooling, esp. with the additional fans installed. Also, the Antec 900 is a very busy case: lots of silly lights. And is a 650 going to be enough? Just wondering here.
Two problems with the 300. The black model has no front fans. The 300 illusion is the model with the two front fans which gives it the excellent front-low to back/top airflow that the antec gaming cases are noted for. You could also look at the Coolermaster HAF 922.
If you get a 5870, the Antec 300 may need some minor dremelling. There is about 11 inches from the back to the HDD rails and the 5870 is 11 inches long. There would be no problem in a 900 or HAF 922.
Since you are going with the 5870/5850 a 750W PSU is probably a better idea. Might see if there is a combo with your case and a 750W PSU.