Yashorton :
I'm upgrading my graphic card to Amd radeon hd 4870 x2.But i have a 450watts psu.The graphic card needs a 650 watts psu . Can anyone suggest me which company psu should i buy?
It really depends on the rest of the PC since CPU's also have a wide range of Watt usage across the board. One 650 Watt PSU should be fine in most cases yes...
Any PSU with Channel Well, Seasonic, Enhance, FSP construction and others should do well (some better than others):
This means most of the following brands: Corsair, Antec, Cooler master, EVGA, XFX, and others (Nox and Tacens in europe are fine too depending on model). Since there are a lot of brands in PSU and I've only mentioned the tip of the iceberg the best you could do is ask if one particular unit you like would do, and I will answer.
By no means I have forgoten other good brands like super flower, I'm just mentioning some brands.
You should never ever build a Panaxuanic or SQNY one, that's the best rule.