"Definitely go with the 27" iMac. Also, about the Mac Pro well, currently the i5 iMac has far better graphics than the Mac Pro. If you don't believe me, go lookup a bench mark haha."
Anyway, your best bet would definitely be the iMac for what you are planning on doing, especially since the iMac has far better graphics, and of course, the harddrive is 3.5", as opposed to the slower 2.5" harddrive found in the Mac Mini."
So...where to start...I suppose the comparison between a 27" iMac and a Mini speaks for itself...
But, as far as the current Mac Pro being bested by graphic card performance in an iMac? Irrelevant, IMO (We aren't, after all, playing Crysis...we're editing/encoding/compressing video). In a Mac Pro....You'll always be able to upgrade not only your GPU, but your hard drives (4x), Disc drives, RAM, monitor, or any type of video capture system (HD-DVI, Matrox, etc) through your expansion slots. Certainly not the case with an iMac.
Not to take ANYTHING away from the new i5 and i7 rigs, they're phenomenal, but not a replacement for a professional video editing computer and it's expandibility. IMHO
And yes, Scratch drives are still very much a mandatory part of video editing (if you want to get it done today

)...with an iMac or laptop, you can easily use an external FW800 drive for scratch, it's definitely quick enough. With a Mac Pro though, you can use one of your 4 internal hard drives as an external...one for apps, one for media, one for...you get the point

I run a Video editing business and I was editing DVCPro HD footage and AVC on a G4 Power Mac before my current rig ('08 Eight Core 3ghz/14gb RAM)....and while it took a lot longer to get anything done, it CAN be done!
In any case, with AVC and other H.264 codecs, many times it's much easier to transcode to a ProRes format (working within FCP) before editing. This is less processor intensive and you'll be able to play back without rendering even with processor heavy effects and transitions.
We shoot with HVX200a and EX-1r cameras as well as Canon 5d2 and 7d cameras. DVC-Pro HD, XD Cam, etc are much easier to play with immediately than the H.264 stuff, even on a monster machine. I always transcode with compressor (It's a fast process) and then edit. Saves a lot of headaches....and, in turn, does NOT mandate you have the fastest machine on the planet. You can do this with Cineform and other programs on the Windows side as well.
To believe you have to have the latest and greatest processors to edit video is simply wrong. If you did, we wouldn't have been editing video digitally until now

H.264 and AVC have been out for several years now...and we've all been doing just fine (albeit...things do get faster

)
J