Graphic card

Well, for your system, i guess HD5670 pci-e 2.0 version will be enough...

indeed. That would be the most you'd be looking at for that resolution.

Will the 5750 work?(consider my psu=250w(18a on 12v))

^^^

Not recommended.

I might work, but you will definitely be putting a high load on your PSU. The higher the load, the sooner the PSU will fail especially if it is generic (like yours). When a PSU fails it can potentially kill other components in your PC.

Which components? That's the fun part you need to test each and every component. Sometimes it can be none. Sometimes it can be all. Or somewhere in the middle.

5670 then.. 512mb or 1gb for my system. i would like to save some money if there isn't much performance diff.

can somebody compare the two versions of 5670 i.e 512 or 1gb

At your resolution of 1280x1024 you don't need 1GB of VRAM, 512MB should be fine.

The amount of memory wouldn't affected much on the HD5670 level...
Save your money and get 512MB version.
And make sure you buy a HD5670 2.0 version and not 2.1 version...  

Just took a quick look around and found what you're looking for, it goes along with what I said.

http://www.bit-tech.net/hardware/graphics/2010/01/14/at...

Notice at 1280x1024 the FPS stays the same, and at 1680x1050 there is a slight difference.

umm... Udai, you shouldn't update your card yet, as Jag said, you need to change your PSU in for at least one with 450-500 watts. Any of the 5xxx series recommends a 400 watt but go higher, you never know. Also, your mobo is a bit outdated   i must say that even updating the card. You will see performance hold backs. As your mobo only has a 16x/1x PCI express slot. Although 5670 is backwards compatible with the old 1.0 16x slots. You'd still see a drawback. So be sure to get the 16x diamond 5670, or this one. http://www.newegg.com/Product/Product.aspx?Item=N82E168... The reason being this card especially runs on Pci express 16x 1.0 oh btw i don't think there are anymore options for you other than the 1gb card for pci 16x express 1.0.
Sorry for any errors. Hope it helps. It is a bit on the expensive side though just saying.

yah i had an old geforce 9600 and a 250 watt psu and it blew out one day ...i would have never guessed that my psu was the torpedo that brought my ship down.....