i really dont have any knowledge about power stuff...here is my PSU spec. http://c-youngtechnology.en.made-in-china.com/product/TqWxruoPIaYc/China-Power-Supply-ATX-550W-.html
i don't wanna upgrade my monitor...
so i buy new one?? and one more thing i need to be on safer side..in future if i want to crossfire wht would be the power requirement??
For an HD 6850 I wouldn't trust that cheapo model. Looks like 16 amps on the 12V rail...Yes, you should buy a new one for a graphics card like the 6850.
For future crossfire I suggest a 650W PSU, again from a reputable supplier like Corsair, Antec, Seasonic. There are some decent models from Coolermaster in that range, but expect to pay about $100 US for a decent 650W PSU. If you prefer something that can handle even more power hungry cards, a 750W will run you about $130-$150. I think you'd be fine with the 650 though.
If you don't want to upgrade your monitor then there is absolutely no need to crossfire. The 6850 can handle anything you throw at it at that resolution. Your needs are simple, with no plans to get a bigger monitor you can easily play anything with one card and a 500W supply.