get a 500W..you don't want your system to die just because your too cheap to afford 10-30 dollars more on a decent PSU .
make sure your PSU has a 6pin connectors, the 5750 needs that.
"The graphics cards of the 5700 series are expected to be released in October, says anandtech. AMD's Hemlock dual GPU cards are supposed to follow in November.
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
450 Watt or greater power supply with one 75W 6-pin PCI Express® power connectors recommended (600 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode) "
although it says 450 or greater, a 500 is less risky and make sure to get a 80+ certified PSU
No one will be able to tell you. I would guess that a mobo can uses anywhere from 25w - 60w depending on if it is a value, mainstream or high-end. The more features the mobo has the higher the potential power draw.
The single best way to reduce power consumption is to buy a 80+ certified PSU.
80+ (basic) = At least 80% power efficient.
80+ Bronze = At least 82% power efficient
80+ Silver = At least 85% power efficient
80+ Gold = At least 88% power efficient
Gold might be off a little 'cause it's been a while since I saw the specs for that standard.
It's hard to find a power rating for just the MB. The power to the mother board depends on the CPU, graphics card (power to the PCi-e slot) and as jaguarskx pointed out all the features - integrated sound, Nr of USB ports, ethernet.
Don't skimp on the PSU I generally recommend a PSU that is double your max load. PSU ratings are for all rails (+5, +3.3, +12 V ect). The primary power consumption is on the +12 V. Also The voltages are more stable and the PSU will last longer if you are not pushing close to max. My best guess is that at max load (Gaming) you will be close to 350 W plue/minus 50 Wats. I would recommend 550 Min and if you can afford it a 650 W. This leaves room for upgrade (ie more Hard drives - They do not use alot) and overclocking (if OC is your bag)