Provided your other components aren't too OTT (I ran my rig with 4890 off a HX450 to give you an idea) , and your 400W is a decent PSU, then you could get away with it. If it doesn't, then a Corsair CX430 or the Super Flower 450W are ideal choices for a 5850 equipped rig
I might try my 400W PS but I really don't think it will work.
I if not I might go for this Super Flower 550W since its only €4 more expensive than the 450W.
Does anyone know how much P&P is from that hardwareversand site? I cant seem to find it in English.