The GTX 670 graphics card twins low power use with ultra-high performances. However, we'd recommend you look for a model with a modified cooling system, as the standard Nvidia module makes an unpleasant noise. Note too that some brands now sell overclocked versions of this card with performances equivalent to or better than the GeForce GTX 680, and all for a lower price!
Best of the rest
Nvidia GeForce GTX 680 2 GB
This is an excellent graphics card. Gaming performances are good, power use is relatively low and it's quiet to run. You also get Nvidia extras like 3D Vision, 3D Surround, Adaptative vSync, etc.
AMD Radeon HD 7850 1 GB
You don't have to spend a fortune to get a graphics card that's decent for gaming. With this model, you get good performances and reasonable power use all at a competitive price.
AMD Radeon HD 7770 1 GB
You'll be able to play the latest games with this budget graphics card, although you'll probably need to turn down the graphics detail. All in all, it's a good tradeoff between price and performance.
Nvidia GeForce GTX 660 2 GB
This is a well-rounded graphics card with decent enough power use and gaming performances. However, make sure you pick up a modified model (e.g. MSI N660 Twin Frozr 2GD5/OC), as cards using the standard cooling system (seen in the Nvidia reference card) are way too noisy.
Top of the Range
Nvidia GeForce GTX 690 4 GB
This is an exceptional graphics card with monster performances, top-notch build quality and a high-end price tag to match. It's clearly reserved for a privileged few, but it's great for HD gaming over several monitors or in stereoscopic 3D.