The well-known market principle often stated as "faster, better, cheaper" also applies to power supplies, though added capacity often cancels out cost savings to deliver more functionality today for the same costs paid for less functionality yesterday, instead of driving absolute costs down. Small but potent PSUs can crank out up to two kilowatts of power nowadays. As a consequence, efficiency ratings for PSUs also continue to improve, so that we can use more of the juice we must pay for to actually get something done.
That's also why it's a good idea to draw up a general power budget for a PC build before purchasing a power supply, rather than relying on the sometimes misleading ratings that vendors assign to their PSUs. You can do this by adding up the total energy draw from each of your system's components. CPUs typically fall in a range of 35 to 130 watts, the motherboard from 25-50 watts sans RAM, drives usually fall between 15-20 Watts apiece, and graphics cards may require anywhere from 30 to 200 watts depending on the specific make and model in use. Add 30 percent to this total when you're finished just to be on the safe side. If you want to make room for future components or upgrades, bump this fudge factor even higher, but don't forget that power supplies tend to be somewhat less efficient as loads increase.
Heavy-duty power supplies get expensive pretty quickly, and in view of quad core CPUs that will impose widely different power draws depending on their energy saving regimes (a la SpeedStep and Cool'n'Quiet) switching individual cores on and off, we can only recommend extreme models when they are really necessary.
The big impetus for power supply makers going forward should not be to build ever-bigger and -beefier units, but rather to keep improving efficiency ratings. To be sure, power supplies rated at 600 watts and up have legitimate uses, but the total population of users who need that much power is miniscule comparison to the legions of average users. With a little knowledge and some considered calculations, savvy buyers can save money on both power supply purchase and operating costs.