An HD 6850 requires a 450W PSU with at least 25A on the 12V rails and (1) 6-pin PCIe power connector; amps on the 12V rails is more important than watts. The PC P&C MKIII with 30A on the 12V rail is sufficient for your build and is made by outstanding OEM: Seasonic.
Seasonic PSUs are amazing (that's just a rebranded Seasonic PSU) but 400 watts is probably below what I would consider a safe minimum. It should work but you'll be running over the advertised max output load when doing some heavy gaming for sure. This could void the warranty on all of your components and potentially cause damage, be careful