Suggest a graphic card for Xtreme Brand 400 Watt Power Supply

Without knowing the specific model of PSU its hard to make a definitive suggestion.
400W is a pretty small PSU. If it's a crappy one that figure could realistically be under 300W. If it's really crappy, you don't want to use anything like that kind of power  

In terms of graphics cards, the best graphics cards you can buy that only use 1 PCIe 6pin power connector are the HD7850 and GTX660. Both of those cards have a recommended PSU capacity of 450W, which is usually overstated but if the quality of your PSU is questionable it's probably best to adhere to it.
In the middle ground there are the HD7770 and GTX650Ti, both good cards that also use a single 6pin PCIe, but both cards still recommend higher power capacity.
At the lower end there are the GTX650 and HD7750. The latter doesn't even need a power connector, and both draw very low power. I'd imagine that either of these would be 100% safe.

A good entry level PSU will power any of the cards I just mentioned without any issues. Where you are actually capable of landing is really dependant on what PSU you are using and how confident you are in it.

Thanks for the reply.I don't think it's a really crappy psu but it is pretty average considering big brands.I am gonna go with hd 7770 as it is a performance card.BTW can you also help me choose the brand??? there are three famous ones here HIS ,Gigabyte and Sapphire(which tend to be more pricier).Which one would be best?

If you can find out the name of the PSU, model number etc, or at worst the sticker on the side of it, it'll provide a much better picture of where you are at.

HD7770 is a good entry level kind of card and not a big power hog, unless you are trying to run stuff at super high resolution/detail levels it'll perform pretty well. It's still a pretty low priced card though, so you are unlikely to see big differences between manufacturers. I'd just go with any reputable brand at the cheapest price, as long as they have good warranty history.

Rammy nailed that one. Haha that avatar.. Oh and HIS brands usually offer better cooling and higher stock speeds + more room to overclock as those huge fans on HIS cards ensure that the card is being kept cool

Genuinely a promo picture on an Amazon.co.uk power supply. Guy on here was asking if it was any good. A picture is worth a thousand words.

Asus 650-E, doesn't need external power connector.

Oh .... I thought I already did ,the name of the Power Supply Company is Extreme\Xtreme and on the sticker is written :"Xtreme Brand 400 Watt Power Supply".I failed to find the existence of such a power supply on the Internet.

Asus 650-E isn't available here .

Yeah that's not really enough information. The sticker on the side of the PSU will provide a certain amount of info (should look a little like this http://www.legitreviews.com/images/reviews/1166/xfx_850... ), but in general if you can't easily find info about a PSU, its junk. Good, mediocre and even pretty terrible manufacturers are very keen to tell you about the many acronyms their products provide. A lack of info is never a good sign.
The reason I'm making a big deal about this is that a lot of cheap or older design PSUs are intentionally misleading in their power output. All that really matters these days is the 12V supply, and you'd expect a solid PSU to provide about 90%+ of it's stated power in 12V. So a reasonable 500W PSU you'd expect to provide at least 450W on it's 12V rail(s) (~37A).

In the real world, you are unlikely to draw much over 200W with that CPU, even if you were using a GTX660, so superficially 400W seems like plenty. But given I have no idea what percentage of that 400W it is capable of supplying at 12V, or how accurate the information it provides is, you can understand my hesitance to suggest something that has the potential to cause you problems.

Hey rammy just to piss you off i'll have you know that i'm running a 9600GSO on a 250w PSU   .. I've cut off the black wires from some other stuff and given it a 6 Pin power connector. Of course my games are freezing and i'm scared to even power the damn thing on until i get a better psu. Can i mail you cause i need advice with something and i really don't wanna start my own thread