It seems you can easily get 400mhash per gpu with 6990 stable, but with 5970 i am only able to get 325mhash OCed to 800mhz. 850mhz gets you 350mhash, you need 900mhz (which is very difficult) to get 400mhash. Also 6990 will resell much better down the road to gamers vs 5970.
Use modified phatk kernel here:
http://forum.bitcoin.org/index.php?topic=25860.0flags: -k phatk VECTORS BFI_INT FASTLOOP=false AGGRESSION=11
At 835oc (which is very reasonable) you should see around 380mh/s
phatk is optimized for 5870 and the 5970 is two 5870 gpus.