Hybrid Crossfire only works with the ati 2400xt, 2400pro, 3450 or the 3470 theres is really no point in getting the 780G chipset if your gonna get an add on GPU. But on the other hand its only $10-20 more for it. and maybe the 780G have more feature than the 760G ?
I would definatly go for the 780G, because it can be used as an HTPC without the need of an add-on graphics
As for system performance, an ATI 2600XT/nVidia 8600gts will dominate the 780G + its hybrid fire

.
The overall performance will depend on your CPU, GPU and rams