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.
Ok thanks, though you still didn't tell me what your budget is. I will assume that $170 is your complete budget for everything.
Your CPU should be good enough for a HD5770.
Your RAM will limit your performance so it should really be upgraded, your choices are adding 1GB or 2. You may have a 32 bit OS so adding 2GB for a total of 4GB will only give you 3.3GB of useable RAM. It has also been shown that 3GB is enough memory for all "normal" uses. Also 1GB sticks are half the price of the 2GB ones so you save a fair amount of money.