Hello, glad that was useful.
In your PC is a PSU (power supply unit), it converts the AC input from the wall to DC and outputs a few different voltages to the PC. These voltages are 3.3V, 5V and 12V. A power supply has a "max" amount of power (measured in watts) it can put out. The typical PSU ranges from 350W to 750W, then some high end ones can run up to around 1500W. The more the power the more you can expect to pay. A normal PC without a big graphics card can run easily on 350W and a PC with a single mid range graphics card can run generally on 500W. Prices range from around $40 to $100.
OEMs (Dell, HP, etc) use the smallest PSU they can to save on money. Why would you use a PSU that has more power then in needed? This is not good for upgrading (which they do not care about). Also the PSUs they use are bad quality.
When buying a PSU you MUST get a quality one. These are much more expensive though they are worth it and you should NEVER buy a bad one. Good PSUs can be hard to pick as even some of the good brands make bad PSUs. The size PSU required depends on the GPU mainly.
Could you tell me what CPU you have and how much RAM as these could be a bottleneck to your system and thus upgrading the GPU won't increase performance overly. Also what resolution do you run at.
For your budget the GTX 460 768MB is the best choice, it costs $169:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125334
Though the HD 5770 is a very close contender and with a nice price of $120 it has much better value. I'm not sure on your use but I would choose this one, it also allows you to get a PSU.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161338&cm_re=ati_hd_5770-_-14-161-338-_-Product
Was the $170 budget for a new PSU as well? If so then you will have to go with the HD5770 so that you can afford a new PSU. Do you need me to recommend one?