You can be 100% sure that if HP sells this directly, the power supply will be enough for the system plus at least 10% headroom. It's been engineered and tested FAR more than any homebuilt system. It may not have enough power for high draw upgrades though but HP (or anyone else) can't and won't account for that.
The answer to your question is yes. The official requirement for the ATI Radeon 4650 video card is a 400 watt power supply. That recommendation is for an entire pc system. You should not have any problems.