What should be the ratio of intake and exhaust fans?

I don't go by any ratio, I just make sure my case is sucking in air and exhausting air out.... I have 2 front 120mm fans that run at 2000RPM, a back 120mm exhaust also at 2000RPM and a TOP 140mm that runs at 1100RPM....

I would not worry about the ratio, I would focus more on the cable management and making sure there is nothing obstructing the airflow.

More than the amount of airflow it is the direction of the airflow - you want the air coming in from the front\bottom of the case and exhausting from the rear\top of the case (since warm air rises) - so you do not want to place an exhaust fan at the front or bottom of the case or an intake on the top or rear of the case as that will interrupt the directional flow causing pocket of warm\cool air to wind up cycling in the wrong direction causing pockets of poor cooling

I don't go by any ratio either. There are too many other factors involved in cooling. I monitor a variety of temperatures while conducting torture tests. If I make adjustments it is for the other factors rather than a ratio.

Front intake fan seems to be a popular choice. If your HDD needs no cooling, is it still necessary to have intake fan up front? Isn't it better to draw in cool air from the bottom of the side panel so it cools the video card area?
In my current set up I have a intake fan over the PCI slots and one in the upper back just behind the CPU, and has 1 exhaust fan on top.

Front intake fans are a must, well at least for me. In most cases the front intake fan/fans cool the HDD's since they sit right in front. Regardless if your HDD needs cooling or not, a front intake is beneficial for overall airflow.

I have no front fans with a Antec 300. 3 120 exhaust total. I have the top 120 exhaust and the rear exhaust/h50rad/120pushing. This creates a good pull through the front where there is a mesh filter. If i put a paper towel up to the front it sucks up and stays there (satisfied me). Everything seems to stay cool. I have a 4770 which runs very cool , so that probably helps. The p55's have no real chipset heat either.

Yea, well with an H50 the front intakes are really not needed. When your CPU is cooled with a sink it is recommended to have at least one intake fan...

I have found that case ventilation needs vary greatly with the hardware, obviously. Vidocards seem the be the dominant variable, where the more heat the GPUs give off inside the case, the more beneficial side mount exhausts are. It is always essential the have good evacuation of warm air from the CPU HSF, whether from the back or top. Front mount intakes always make things better, heh.

Before I installed my 4870x2, I didn't use a side mount fan, just 120mm fans in the front and back, with a loose funnel vent to the CPU HSF and an open grate over the PCI slots. The x2 produced enough heat to warm my CPU dangerously during gaming. I added a 120mm side exhaust to the grate and the problem dissappeared.

When I upgraded the stock CPU cooler, I rigged a direct airflow tunnel to my CPU HSF (top mount fan). This is also around the time I replaced my 4870x2 with my original 4870 512mb. I left the side exhaust on, and noticed a very healthy drop in my GPU temps compared to before I had it. This 4870 now idles at 44c, before would idle at 55c. I have yet to see it break 80c during gaming now, and before it would often breach 90c.

There is no solid formula for ventilation, each build/environment is different, but it never seems to hurt to go for more.