Monitor Size & Resolution

Which is better for gaming: IPS or TN, and what size is best? If you say one or the other, please provide the monitor you think is best for me. I will be playing Runescape, Skyrim, and BF3 mainly. I can spend up to $270. I will be using an i5-2500k and a Radeon 7950.
Thanks.

Will also watch youtube videos. My preferred resolution is either 1920x1080 or 1920x1200. Idk which one is best.
Also, what IPS should I choose if you say to go with IPS?

My preferences are:
Low input lag
Nothing higher than 1920x1200
High Pixels Per Inch
LED Backlighting preferred but not required
Good contrast ratio
Can use Portrait and Landscape mode