I agree that you ONLY need a graphics card if you wish to play any demanding games. The Intel CPU graphics can handle some very old games or new ones that aren't very demanding.
If you wish to play any modern video game I recommend the GTX 560Ti.
You do not require a graphics card for video editing. It can speed up things a little but unless you do that a LOT it's not worth it just for occasional video editing. Also, video editing quality is ALWAYS better when the graphics card is not used. That's 100% certain, though it will change in the future with better software.
Don't buy a graphics card yet. If you play games and want better performance get a GTX560Ti. I like the ASUS version.
Just like explained by Photonboy, get a discrete card only if you plan on gaming with the rig. Pretty much the rest of it can be handled by the on die video chip. Though keep in mind that the i5 2400 has the HD 2000 video chip which is somewhat inferior to the HD 3000 found on the k suffix sandy bridge processors.