That's an excellent question. Unfortunately, there are no technical reviews because the new ATI Radeon HD 5000 series graphic cards are not yet available. The cards probably won't be available until October. The only thing I saw were press releases indicating the new gpu's will be 40nm like the ATI Radeon 4770 video cards. If the cards are also energy efficient like the 4770, then the general rule of thumb applies. A high quality 500 to 550 watt power supply can easily handle a system with just about any single video card made.
My crystal ball is broken. I cannot predicate what computing will be like 5 years from now.
The GTX 275 maxed out at 140 watts. The general rule of thumb still applies. A high quality 500 to 550 watt power supply with sufficient current (amps) on the 12 volt rail(s) can easily power an overclocked system with a GTX 275.