That really depends on what you will be doing... Playing games? Photoshop?
If you want something low cost though I would go with one of the following cards:
XFX HD 4870 750M 1GB DDR5 DUAL DVI TV Out PCI-E Graphics Card
OR
HIS HD 4670 IceQ Edition 1GB DDR3 Native HDMI DVI VGA Out PCI-E Graphics Card
The first one is low cost/high performance.
The second is lower cost/basic performance.
There are some cards in between too. Like the HD 4770