Someone please interpret the iMac graphics performance charts

this is the info in reference:

http://store.apple.com/us_edu_444120/browse/home/shop_mac/family/imac?mco=MTE3MDQ

I'll be doing video editing in 1080i of home movies from Canon HV20, miniDV. eventually using FCE but probably with iMovie in the beginning.

I would like to know how much faster, in hour/minutes/seconds, will my HD stuff render with the better graphics cards & discrete memory, everything else being equal.

Is it really worth spending an extra $180 to upgrade to ATI?

No one here can access the link you have provided because it is in the edu section of Apple's site.

But if you want to know about different video cards, do a google search for "abcvideocard vs xyzvideocard".

Also search for "abcvideocard reviews" and "xyzvideocard reviews".

You should be able to find reviews along with timed tests for encoding along with benchmarks across multiple apps.

You can also check these sites:

PassMark Software - Video Card Benchmark Charts
Performance Charts Graphics Cards - Benchmark

and others.

Conveniently enough, I can as I work for a school and have access to the educational section.

He's looking at the educational version of this page:

iMac - Apple Store (U.S.)

the only difference is pricing.

I'd definitely look at the benchmark information, but I'd also do some searching on HD4850 final cut express, or a vs for video editing of the three main card options. I'm using an older 8800gt in my mac pro, and it works wonderfully, but I can't honestly say where the 8800gt falls in terms of the nvidia chipset options available (aside from much higher then the 9400m )

In terms of an hours/minutes/seconds comparison - you'd have to have two systems, identical in every way (configuration, any additional software running in the background, patches applied, etc.) except the video card, working with identical footage to get a good comparison. On top of that, some things that can affect an opinion is the length of the footage - longer footage will, obviously, take longer to render and/or export. Once it's in AIC (Apple Intermediate Codec, in iMovie it's transparent, you don't have to select anything to work with it - in FCE you just have to select it for your sequence and capture settings) - the amount of rendering and what rendering will need to be done will be heavily dependent on what you have done on your timeline. Some things will require little or no rendering, where as some effects may require to be rendered before preview let alone export.