But the hd 5670 supports 3 monitors of high resolution,But the 4850 dont!
Do you plan on using 3 monitors? If so then the 5670 is far superior than the 4850. The 5670 is not powerful enough to drive 3 1080 screens in eyefinity gaming. For the cost of a new 1080 screen you could easily get a new 5770 or 5830 (when it comes out). Frankly getting a better graphics card would be a bit higher on my priority list than getting more monitors that your current card cannot handle.