No Problem, the Newbie Section is for things like that.
Well, i use both (My Server, and my Main PC) with Crossfire, and i cant talk about performance loss.
I have in main Main Both Cards 1000/1200 Crossfire Enabled 450mh/s per Card (1.274 On Water)
In Server , Both Cards 950/1200 Crossfire Enabled 425mh/s per Card (Stock Voltage On Air)
To Enable Crossfire, you have to attach a Screen while Starting (Or your Dummy Plug)
And then Connect Via TVNC to control it, this is what i do.
When Crossfire is enabled, you just unplug the Screen/Dummy.
I Uploaded the File here:http://www.mediafire.com/?r16a4femd9x37ez
I also wrote a Small Installer (Installer.bat) , run it, and poclbm is at C:\poclbm,
and a batch File to start it is at your Desktop.
(You have to add your host/port/user and passwort to it first, to make it run)
But Pararmeters etc. are the best for 5870.
Some Cards are Better for Overclocking some not, i can hit my Cards unter Water over 1100mhz core.
But when i open any Site that contains Flash, it crashes
Hope i could help you a Bit.