Hybrid crossfire is crossfiring with the onboard graphic and it is mostly useless outside of perhaps laptops. To crossfire HD4870s you will need a fully crossfire capable motherboard that has 2 PCIEx16 slots. Some motherboards that claim to be crossfire and have two slots have the second slot actually function at x4 speeds which severely limits the advantages of a second card. So it is only a good idea if you have 2 slots and they both function at at least x8. When using 2 cards with a different amount of memory they with both function with the lower amount of memory(512mb in this case.)
i have 2 crossfire slots.. and one is x16 other is x8 so if i connect second they both will work as x8 i know that, but like i said is 512 mb and 1 gb difference is an issue?