well, i had the Cooler Master Real PowerPro Modular,700Watt for 100 euros, and that is more wattage for the same price, and it is modular, which is really handy.. :D
you need more than 620 watts to run 4850s. 700 or 750 is good. if you have a 620 watt, it will always run at like 100% load and that will die really soon. if you crossfire dont do corsair, do thermaltake, they make good psus.
i have a thermaltake tough power, its amazing. also ive made two friends buy corsair's the 1000watt and 550watt, both which have been nothing short from flawless.