Dell seems to underrate their power supplies.
Case in point. When I ordered my 435T computer I got it with a graphics card that required at least a 500 watt power supply. The 435T came with a 475 watt power supply. I questioned this and was told that Dell tests every configuration and not to worry that I can add another HD, max out the ram and still be OK.
I have a friend who owns a computer repair business that came over with his test gear and measured the PS. It measured a constant 531 watts.
He also told me that most if not all graphics card manufactures overrate their cards power requirements.
You should be fine with that setup.