As Black Zion mentioned prices are very high for GPUs cards at the moment.
But once you decide to purchase another GPU card you need to consider your CPU and PSU also.
Depending on the GPU Card you get and the CPU you have installed will determine how much the CPU will throttle the GPU (performance). Black Zion can probably tell you which GPUs will work best with your processor.
But the most important thing is the PSU. You need a PSU that will be powerful enough to run your Computer and GPU card without power issues.
Once you decide which GPU card you want, you can go to his GPU/PSU website which give a good reference to the type of minimum PSU you will need to for various GPU cards both AMD & Nvidia: http://www.realhardtechx.com/index_archivos/Page362.htm