For graphics cards, ventilation mostly depends on the fan mounted on the video card itself. However, the case's overall air ventilation also affects the temperature.
Cases are tricky business - the prices mostly depend on personal taste of looks, and because they are simply just a metal box, you can always modify it to your own tastes.
However, if you are looking for ventilation tips, I could give some good guidelines. To get good airflow, you would want a case with a good air intake in the front of the case, and a good air exhaust near the back of the case. (Current power supplies rated with ATX standards also act as air exhausts.)
Personally, I found 120mm fans with large a CFM (cubic foot/min of air moved) makes the best type of air exhaust in most cases.
If your case can fit it, a single 120mm fan mounted as an intake on the front of the case also does wonders. If not, I would go for a case that allows optional mounting of 80/92/120mm fans in the front for accelerated cold air intake.
For extra ventilation, you also might want a blowhole (an air exhaust on the top of the case) and a side ventilation system (an air duct over the CPU fan on the side of the case used for exhausting CPU heat).
If you have a large budget, the Antec Nine Hundred is a good choice. For around $130, Antec overkills cooling by adding a ridiculous amount of high CFM fans to the case. Although it's very nice, I personally wouldnt pay over $100 for a case.
For the average budget, almost any case will do. You can always install fans or drill new fan holes onto your case

. But for me, my personal recommendation is the Cooler Master Centurion 5 - newegg currently sells it for $40 for the one without a transparent side window, and $50 for one with. Although this case doesnt come with a top blowhole, you can also cut one out yourself

, and $40 for a solid case is always a good buy.
My case right now is the Apevia X-Cruiser. It is currently $60 on Newegg, and has a blowhole on the top and two 80mm intake mounts in the front. The case comes with only two 80mm exhaust fans, but I went out and bought a Silverstone 120mm fan to replace it.
Remember, cases also depend largely on looks - these are just guidelines to my personal choices, but it's best to go out and choose one that has these features with the appearance you prefer
