Guys i want to buy a new graphics card and i want to know which one is best for

2GB is too much for your build. It will be bottle-necked by your CPU and motherboard, and wouldn't be able to work at it's fullest.
You need a "1GB"-level video card, that's the highest how you can go with your current build.
I can recommend you a couple of GPUs depending on what PSU you have.
Also, what OS do you use - 32 Bit or 64 Bit?
Because if you're on 64 Bit OS it is better for you to get more memory (your motherboard can support up to 4GB, but 32 Bit OS can't see more than 3.3GB).

i am using Windows 7 32 bit

Please tell whole pc specs

As I said before, first tell me what PSU you have. At least tell me it's Wattage.

450 watts power supply

450...hmmm.....

Well, you can definitely get HD 7770 or GTX 550 Ti.
If you'll do decide to get HD 7770, get HIS' HD 7770 iCooler and you'll be absolutely fine.
It's very power efficient and performs greatly (for a card of "low segment"-level) at the same time.
I'm not recommending any particular GTX 550s mostly because it's quite weaker than even the most weakest HD 7770s, while it costs almost the same (or even higher, in some cases).
So you should get it ONLY in the case if you want Nvidia's card and no other, which I personally wouldn't endorse, since that will be fanboyism, and I'm not on either of sides in this war.

best for your system is HD7750, no need to go beyond that, get the cheapest one regardless of any specific brand.

HD 7750 would be performing somewhat worse than HD 7770, while costing just a tiny little bit (almost not significant) less than HD 7770 and being tiny little bit less power hungry.
So it's a bad option in my opinion (which is always based on my personal experience of dealing with products). Just not performing good enough, while it's price not being low enough.
Why buy something's that's quite worse, when you could buy something that performs quite better while costing basically the same? HD 7770 is the "bang for the buck", HD 7750 isn't.