You could do 6 cards by using PCI-E extenders. Just a warning though you run the risk of exploding the motherboard if you run 6 5870s, as the PCI-E bus still delivers power to cards despite having external power connectors. I don't know the exact numbers, the spec calls for 75W max, so say 40 * 6 = 240W running through the PCIE Bus.

Of note, a $200 motherboard that requires a $200+ CPU isn't necessarily that money saving. You can find cheap AMD AM3 boards with 5 PCI-slots (x1 works fine) for ~$120 with a $40 sempron that saves you > $250.

In fact you could get 2x $120 mobos + 2x $40 CPUs, have 10 slots for cards and still have spent less than you would have on LGA1366.