You can get 330MHash easily using cgminer and only using the config file. Normally you would use the AMD Catalyst thing to overclock but if you have your config file setup properly you can do everything there. There is an example config file for the 330MHash/s rate at http://www.brandonorndorff.com/bitcoin/mining/radeon-hd7850-7870-cgminer-config-settings-bitcoin/
but it seems that it is increasing as he tweaks it. Earlier today it said 300MHash and he has already tweaked another 30mhash out of it. The best part is it it running at only 58 degrees or so.
I'll describe my experience with an Asus 7850, and try to compare with the previous post.
It comes factory "overclocked" to 1000 MHz (engine), default memory @1250.
The AMD CCC allows engine overclocking up to 1050.
OTOH, memory clock can't be set lower than 150 MHz below the engine.
So, in my case it would not be possible to use 1100/900 setting ftom the randonorndorff page, even if I'd overclocked the engine to 1100 with CGminer, which is possible. I'd then have to set memory clock to 950. With my 1050 MHz engine clock, I've to leave the memory at 900 MHz.
Another interesting thing I can see in the linked page settings is the usage of poclbm kernel. Which is not the default CGminer kernel AFAIK.
But it's the default one in GUIminer. When I had tried CGminer with the same frequency settings (1050/900), it had crashed from time to time, while it was pretty stable with GUIminer. One idea was that cgminer's default kernel gets a bit more from the card at the price of a bit higher power consumption, which the PSU can't supply. But it has been neither confirmed nor invalidated.
With the above settings, I've been getting slightly above 300 MH/s from the card.