I have to 5830 in my machine at home and had a few issues getting it to work half-way decent.
With MSI Afterburner, I had to set the EULA agreement in the config file to allow above normal overclocking first.
Then, I needed to setup the overclock on the first card.
First things first, I had to change the memory clock speed to 600, apply and then close the program.
On re-opening, I could set the memory clock speed down to 350 and the core speed to 975.
This is when I ran Phoenix for GPU1 and had it speeding away at 285-300 MH/s
Then changed it to GPU 2 in MSI Afterburner.
Again, I changed the memory clock speed to 600, apply and then close the program.
On re-opening, I could set the memory clock speed down to 350 and the core speed to 975.
This is when I ran Phoenix for GPU2 and had it running at 240-260 MH/s
I believe this is because I was just switching the monitor instead of using a dummy plug.
I am going to be making some dummy plugs tonight, and will test it again to see if I can get 285-300 MH/s
out of the second GPU.

I'm running Windows 7 32bit on that machine BTW.