Need a power supply for hd 6850

hi! i am planing to buy a new hd 6850 for my pc but i have firstly have to replace my power supply...
its 450 watts and came with the pc case and with no name which means its unbranded, so far it hasn't caused problems but i the card wants 500 watts minimum,
my retailer store offers the following power supplies and they are the one i can only buy so plz suggest the suitable power  
http://galaxy.com.pk/powersupply.htm ( 1 USD = 85 PKRS) currency exchange

my specs are
core i3 540 3.06 ghz
4 gb ram
intel dh55hc board
intel hd graphics
and a no name 450 watt power supply