The hybrid crossfire won't work with the HD4850. Hybrid crossfire to the best of my knowledge only works with the HD3450 and HD3470 model graphics cards. I also wished that hybrid crossfire would work with other 4000 series cards, but after a lot of research I found this to not be the case.
Even if you could get it to "work", it would not be beneficial to you as the onboard graphics are quite a lot slower than the HD4850.
For crossfire X you would need to have a second HD4850, and a second PCI-x slot (16X). I am unsure if your board has this, but I doubt it. Your HD 4850 is crossfire x CAPABLE, but unless you have a second card that functionality will not be available.