PowerColor exhibit watercooled Crossfire
Water definitely seems like a popular way to deal with the heat output from Crossfire, and PowerColor's Evo2 system is designed (at least initially) for their Crossfire X1600 XT product. With master-less Crossfire doable with X1600 XT, pairing Evo2 with boards seems like a fine way to near silence with good performance.
One heat exchanger does the hard work and it sits in a PCI Conventional slot. Hilariously, PowerColor's site is down at the time of writing, so more information is hard to come by with the Messe shut for the day. An update tomorrow if poss.
