If your motherboard has an integrated Hybrid crossfire card, could you use a crossfireX card with it?
More abouthybrid crossfire crossfirex
afaik the hybrid crossfire got cancelled, it only worked with HD3600 or 3400 series cards, after that it faded away...
It works with 780G/785G/790GX chipsets and a card with the same number of shaders, namely the 2400XT, 3450 and 3470. The performance of such a set up though is about the same as a cheap 4570 and a little under a 3650.