Well heres the problem with the q9450.. and its also why amongst overclockers it wasn't very popular..
- low multiplier (7.5x)
- Need of really good board if you want overclocks like a q6600 would get
- low FSB Hole
With Crossfire you just put the cards in the Crossfire ready board, attach the bridge and enable Crossfire in the Catalyst Control Center
Yes they can, you use a 3rd Party Software called ATiTool or RivaTuner