New graphic card

Hmm ok, here are a few options:

http://www.cclonline.com/product-info.asp?product_id=62...

The GTS 250 delivers good performance for price.

http://www.cclonline.com/product-info.asp?product_id=37...

Slightly cheaper and with great power/performance ratios (5750)

http://www.cclonline.com/product-info.asp?product_id=61...

The 5770 is more expensive but gives more performance.

http://www.cclonline.com/product-info.asp?product_id=55...

A great card, well worth looking at (GTS 450).

Those are some options to think about, my only concern is your power supply isn't amazing. The above cards will work, but it will be tight.

Having said that, those are the cards that will take you where you want to go with that system of yours.

I would get a 5750 or a 5770 is possible, they are really power efficient than their Nvidia counterpart.

The 5750 consumes the least power, followed by the GTS 450, then the 5770 and the most is the GTS 250.

It's not universally true that ATI cards use less power than Nvidia cards. Some do, some don't.