I run a few 5830s as well. In the GUIMiner...um...GUI, find the "Extra Flags" field (Just under where you entered your miner password). There are a ton of flags out the that can boost your hash rate some but I have found that this works best:
-v -w128 (typed just like that)
Clocked up to 975 core and down to 500 memory I get 300m/hash. Clocked up in the 1050 range is even better if your cards can handle it (two of mine can one...cant).
Once you add those commands in the flags field do click stop mining and start mining again to begin with those commands active. Some may say to change the -w rate to 256 or so. I found 128 to be a slightly better rate on my machine though.
Further flags can be found- http://forum.bitcoin.org/?topic=4122.0