This is a rough hierarchy guide.
If you don't game even a $30 card is sufficient. If you do, the better the card the better your game will run.
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
Diablo 3:
It can run at the highest quality on less than an HD7770.
Far Cry 3:
This is probably the most demanding game and no single-GPU card can run it at the highest settings.
*VALUE:
AMD HD7000 cards have free games (digital coupons). I'd recommend one of these depending on your budget.
Quality:
Not all cards are equal. Some have better cooling solutions and better quality.
Recommendations:
1) about $300:
The following HD7950 comes with two new games (Crysis 3 and Bioshock Infinite) which would cost about $120 by themselves:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202006
2) about $200
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121651
If you get an NVidia card or cheaper AMD card you won't get these games so factor them in if you're interested.
Power Supply:
- ensure the proper 6-pin/8-pin PCIe cables are there
- look up specs (recommend a minimum 450W for HD7850 and 500W for HD7950)
- if you need to buy, look for SALES at NCIX etc. (*read customer reviews to look for issues).
http://us.ncix.com/products/?sku=67324&vpn=HCG-520M&manufacture=Antec&promoid=1387
*A poor quality power supply can destroy your PC or even burn down your house. You don't need the absolute best but don't get some cheap no-name brand. Even if the basic SPECS are met, a lower-quality power supply won't deliver the same amount of power. The "450Watt" on a PSU is the theoretically TOTAL by adding up the maximum each output can deliver. It's not a number you can actually hit. A better quality Power Supply might draw 400Watts whereas a cheap one might only hit 320Watts.
Summary:
- spend about $200 or $300 on an HD7850 2GB, or HD7950 3GB
- two free games (digital download, get the games as soon as possible)
- Power Supply