U can CrossFire a HD 4870 with HD 4850:http://www.tomshardware.com/forum/245454-33-crossfire-faqs
But i dont recommend it :
A note about Motherboards:
The last one is 16x 4x, this one is the weakest and doesn't perform very good compared to dual 8x or dual 16x,so i won't recommend it for Multi-GPU configuration. Intel P35 and P965 chipsets support Multi-GPU at 16x 4x mode.