For a system using a single Radeon HD 6850 graphics card AMD specifies a minimum of a 500 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 25 Amps or greater and have at least one 6-pin PCI Express supplementary power connector.
The BFG 450-Watt power supply should have a combined +12 Volt continuous current rating of 25 Amps and have one 6-pin PCI Express supplementary power connector so it just meets the minimum power supply requirements.
Just don't plan on doing any overclocking though because that will increase the power draw beyond what the power supply is able to deliver.
you have stated the input voltage and current draw of psu from wall, we need the output current at 12v rail and power output at 12v rails.
Read the lable of psu for output current and power at 12v rail.
If this is your psu https://techwiki.hardwarecanucks.com/product/1MDMyMjg/B...
then you got 25a on 12v combined and thus it will work for some time but will damage your psu at full loads.
You can get a hd7700 which is equal to hd6850 in performance and only use around 80w. And it will work withn your current psu.