This is not the understanding I have of it, nor my friend who does it professionally, I aksed him to write a small sum up regarding this, btw, he is recognized by quite a few major lables already and I just helped him build his latest machine. If you are interested, her eis his explaination, I have little time as of lately to decipher it for everyone here atm...
"The most important aspect for video editing is undoubtably the video card. Of course the Processor, RAM & Hard Drive are all major factors, but of course, how can you leave out the one thing that is most necessary in your endeavor.
For those that say Video cards aren't important for editing haven't had much experience in editing. Most NLE's work best with cards that support OpenGL. Adobe After Effects, Pinnacle Liquid, Avid Express & others run best when a video card supports this feature.
Number one rule of course, is how fast you can 'render' your timeline. If you are telling me that there is no difference between a 16MB Video card, and a 512MB card, then I have something else you should look into. The faster the card, the more processes it can spit out per second. That's a whole lot more information faster, and at less stress to the CPU and overall temperature of the system.
Not only that, but certain cards benefit editors with little extras, like support for Plug-ins to NLE's, like Boris FX and what not. Forget rendering basic video...trying creating a new title or animated graphic in After Effects, and see how well your shiny new, P4 Processor with HT Tech. spurts that one out. Be sure to make time for lunch while you're at it. Forget about adding filters to existing video clips in the timeline. You might as well get some jogging time in as well.
Dont forget other perks like Dual Monitor support (and up). I've seen incredible 4-5 monitor support machines. How can you get that type of desktop real estate without a capable video card? As well, for an editor...if you don't have a breakout box, you can always get a new card that supports both Digital & Analog support. From DVI, to S-Video & Composite In/Out, you'll never have to worry that you won't be able to capture/export video you are working on.
Overall, for better system performance, more work done faster, cooler system temp., and more hair on your head, do yourself a favor and get a video card that is more in tune for video editing. "