ok with my actual HD 5850 I reah 395MH/s with 1000/300 at 1200mV (80°C)
I wanted to push a little and reach 400MH/s
You can probably reach 400MH/s by playing with the software rather than pushing the card further. I've found setting which will give me 413 MH/s for a 1000MHz 5850 but you might have to pull that back a bit if your rig is not a dedicated miner. Let me know if you want details.
Do you have the latest miner kernel code? There have been some improvements in the last few weeks.
I use GUIMiner (last version) with those option :
-v -w 256 VECTORS BFI_INT AGGRESSION=13 FASTLOOP=false
Is there something to change?
-w 128 gives me better results with my 5850.