There's a lot of factors to consider when getting a graphics card:
Anything 1GB (minimum) or better
128-bit bus will be fine as long as its GDDR5, if you can/want better that 128-bit, go for it.
Your current Power Supply (look at the 12v rail, it will say what you have for amperage)
So when you find the card you want research what it needs for amps.
The size of the physical card itself (you're going to want to get the measurement on it before buying one making sure it fits your case)
As for recommendations, look for Brands that are good like: Asus, EVGA, Sapphire, and MSI. Newegg.com and Amazon.com are my favorite places to buy from.
Also this is a link to a chart that ranks graphics card performance: http://www.videocardbenchmark.net/high_end_gpus.html
