i have 2 power supplies.420w and 460w. i am planning to use 460w for sapphire hd5850 and the other for rest of the pc.Will that serve the purpose of hd5850 or should i attach only 1 psu.as the ATI recommend 500+ PSU?
I wouldn't recommend using separate power supplies. Not saying it can't be done but if you are asking here if you should do it, I'd say it's probably not a good idea.
That can cause a lot of trouble and is high inefficient you are better off using a single psu. Use a website such as Thernaltake psu calculator to find the best fit wattage for you. Always remember to keep it as simple as possible less things to go wrong then