I'm tempted to start a thread "Who's running 5830's and getting less than 300 Mhash"
By reading this board, I think there'd be just one post in the thread!
But actually I'm sure there are some others out there. The card's stock settings are 250 Mhash/s and if you have the wrong motherboard (or cooling situation, climate, etc.) the 1st card in the system can get very hot.
I have 1 Asusrock motherboard (the rest were all sent back defective) that has the 2 PCI-E slots very close together.
Turns out there's a much better mobo for $10 more that has *much* better placement of the PCI-E slots, plus it's a better board.
Anyhow, unless you're running Windows, it's also difficult to mess with the card's voltage. All my cards are stock voltage. I'm running Linux.
My XFX 5830 @ stock is 800core 1000mem. When I am @ the pc, it is at 1010core 600Mem @ 313-14 Mhash/s, when afk, its at 1030core and 695 mem @ 324 Mhash/s. For this card I have, does not allow me to change voltages, even if flashed, it is missing a voltage hardware component to allow me to change"xfx idea". Lowest I can go for mem is 500Mhz but then it is not stable.
I will be looking into another card in ten days or so, by then, maybe I'll have enough mined for a 5850, if not I'll go for another 5830 and build up.