I picked up a 5770 cheap yesterday to run along with a 5830.  Noticed 100% cpu usage once I started mining. I had  0% usage when running one card.  Seems to be a common problem caused by the openCL drivers.  Here's what I did to fix it.  Running WIN7 64 bit / Phenom II dual core cpu.

I have one monitor installed to both cards (HDMI / VGA port)
Boot with both cards plugged in.  Go to screen resolution display settings and on the drop down menu for multiple displays select " Show desktop only on 1" (for 1st installed card)
start up your miner for that card.  Go back to display settings and select "show desktop only on 2" (for 2nd card) Now start up another miner for that card.  
I am now getting 0% cpu usage.  I'm assuming this will work for Windows XP also.  You may have to select extend display or detect first for Windows to recognize both cards before doing the above.