A decent crossfire LGA775 motherboard will cost you at least $80. So figure around $200 total with the second card. Another question is if your PSU can handle two HD4850s. If not then you'll need a new one and at that point it's just getting too expensive.
Honestly even if your PSU is fine for crossfire it probably makes more sense to just replace the card. That $200 plus ebaying the HD4850 will get you close to $300 which can get you an HD5850. The HD5850 will give you performance similar to two HD4850s with several large advantages; DX11 compatibility, 3 monitor support, no possible crossfire scaling/compatibility issues and its 40nm which means it's very power efficient and runs cool. It uses the same power as one HD4850.