You're outside of my expertise with this one, but fortunately I'm part of a great professional educational consortium here in Colorado called Boulder Digital Arts, so I sent your question along to a couple of the video editing instructors, Ethan Benninger of Bonefire Films, who teaches the Final Cut classes, and Brendan Boykin, a Certified Master Trainer for Apple Pro Video and head of Creek Mountain Media. Here's what they suggested...

(first off, Ethan)

For starters, here is a useful link to the Final Cut Studio System Requirements (and for a good post-production experience, do not simply go with the minimum possible specs):

http://www.apple.com/finalcutstudio/specs/

I recommend getting the fastest machine you can afford, and make sure it has at least 4GB of RAM (memory). If ALL you are going to do is edit video, then you can get by with a much slower machine; however, if you think you may dabble or even plunge into Motion and Color, then you NEED the faster graphics cards with more video RAM. I HIGHLY RECOMMEND the 15-inch 2.8 GHz MacBook Pro with the available video card NVIDIA GeForce 9600M GT with 512MB.

Motion uses your GPU (graphics processing unit (your video card)) to process real time 2D and 3D motion graphics. If you are going to use Motion, do yourself a favor and bite the bullet now by getting the faster computer/GPU combo.

Color likewise uses your GPU for rendering, and is another beast altogether. It is a very powerful program, and was designed to run on 2 displays. If you think you are going to do a lot of Color work on your films, then consider getting the 17-inch MacBook Pro--15-inches is really too small. At the very least get a secondary display to hook into when doing Color work. This also brings up the issue of color-balancing your displays so that red really is red, blue is blue and green is green... but that is another issue entirely.

Lastly, I would not recommend purchasing a new MacBook Pro in the next month--updates are right around the corner. Here is a good resource to check before buying any Apple product (the MacBook Pros are about 2/3 of the way down the page):

http://buyersguide.macrumors.com/

(now Brendan)

At minimum the 15" laptop the 13" will not cut it when it comes to Motion and Color. If you are serious about using a laptop for editing:

1) Only the 17" has more expandability options with the ExpressCard slot (additional storage options, connection to outboard gear like AJA and Matrox)

2) Editing on a 15" screen (or smaller) sucks. With the 17" running 1920x1080, at least you can see the entire interface of the FCS apps and not be cramped.

FCExpress is a very capable program, however, you are limited to what codecs you can work with. Any AVCHD is transcoded to Apple Intermediate Codec which is old and bloated compared to Apple ProRes. Be prepared for lots of big files. For example, a 10.5 second AVCHD clip is natively 18MB. Final Cut Express will transcode the file to AIC during transfer yielding a media file that is 222MB!. FCP7 allows you to transcode that same clip to Apple ProRes (LT) yielding a 105MB media file. And remember that was only for a 10 second clip. The difference of storage requirements per minute....and then per hour of media....is huge! The numbers will vary slightly as ProRes is a VBR codec, but will still be about half the requirements of AIC.

I hope this info is helpful, happy editing!