I'm not sure thats the whole problem shadow. It sounds like it may be more likely the actual PSU to me as it seems to be under-powering the HDD's and stuff. Its probably worth it at this point to just replace the whole PSU- at that point you could add your other 4850 and run the tri-fire as well. I would just get a goo quality PSU that will handle your ~300W of GPU (with the 4850x2+4850) as well as the rest of your system. I would guess a good 750W would be a solid choice. http://www.newegg.com/Product/Product.aspx?Item=N82E16817139006
Corsair is one of the best.