I am with the above poster and rolli59 on this one. Unless you are into extreme overclocking, your current power supply will be just fine.
Point in case AMD says 450 watt for a 5770(6770). But it runs just fine on a good 300 watt unit(with power to spare). Once again, no extreme overclocking or anything.
But a quick question, will that PSU run a 6850, if im running BF3 for hours on end? (i usually dont play it hours on end..... but like to know if i decide to if it can handle it)