I would get a case only then get PSU depending on what your getting. You can't just buy a PSU and add components later when you don't know what your getting.
Any new graphics card will burn out that PSU. If your getting a 6600GT its fine. The 6800's and up require more power than that.
Get everything AT ONCE (you'll hate yourself when you find out you find out something you bought dropped $50 in price by the time you put the computer together)