next to needing a DAW for my music production, which is admittedly a lot less taxing on CPU performance, I edit mainly HD from either my DSLR or mp4 from the Sony pro range.
Thing is that I really really would like have a better preview speed in vegas, as well as keep my PC from choking on HD source material as soon as there's FX going...
I reckon the better CPU speed will definitely help with that.

If you consider the whole package the i-Mac has certainly an attractive price.

The reason I personally wouldn't go for it is expandability which just doesn't exist with the i-Mac.

Before you get it for use with Windows, make sure you research how the display will handle all those pixels.
I run a similar setup to your current one - a Mac Pro 3,1 with Windows 7 on a partition - and I'm also checking out the Retina iMac. The iMac, though handles the 5K display through pixel doubling - it doesn't actually display the full 5K, although in video and photo windows it does. Usually, though, it's scaling type and other display elements for a much sharper 2560x1440 image. All Apple technology, or rather software. Windows, if it displays a full 5K in a 27 inch monitor, will give you such tiny type and interface it would be practically useless.
Or is Windows 7 or Windows 8.1 capable of similar pixel doubling strategies?

I'm considering the new 5K iMac instead of upgrading my Mac Pro. The price is outstanding considering you'd be hard pressed to get a 4K display without a computer for that price. I was really blown away by the pictures of Final Cut Pro X showing a 4K "preview" in full resolution with enough pixels left over for the FCP X interface. Wow!

If I was working with 4K I would definitely jump on it. I am not working with 4K, nor do I see myself working with 4K in the near future, so I might pass on it and still upgrade to a Mac Pro instead. It's hard to say. I'll have to drop by the Apple Store and see the iMac 5K for myself (or maybe I shouldn't because I'll wind up buying one). lol

~jr

since my wife has taken to doing some heavy duty photography and ps, she's been eyeing my pc...

so i got to thinking that maybe i'd look at the new imac too....

http://www.extremetech.com/computing/192425-the-new-imac-with-5k-retina-display-vs-mac-pro-which-one-should-you-buy

as it is, to pimp it out for work (i mean an i5 isn't an i7 in vegas), i'd be up for $4k / $5k aus respectively.

i've recently bought a catleap hd monitor 2560x1440 for $300au (guaranteed no dead pixels), and it's great, and as i'm unlikely to even look at 4k video in the foreseeable future, a 5k display might be overkill ;-) - as an aside, i like discrete hardware, aio scare me, one bit goes wonky and you're dead in the water. i also like flexibility in my hardware, such as swapping things out, such as video cards.... so i'm a bit iffy overall about macs, but i do like their design and hardware.

so, what would (split the difference) $4.5k aus buy me in australia? i haven't researched this yet since she's only eyeing my pc, but other than equally well spec'ed hp / lenovo workstations to the imac, the diy looks even more promising.

as it is, i'd wait till there's some respectable reports in on the imac before doing anything ;-)

I just visited a friend with a slightly older imac. The first thing that bugged me was the noise, then the lack of connectivity and general flexibility, as people have already pointed out. My current machine is actually in the next room, connected to the screens and hardware in my workroom - I'm used to it just being dead quiet, and even the slightest fan or disk noise from the imac instantly got me nervous.
Obviously couldn't do that with an imac...and coming to think of it, I guess I won't be doing 4k any time soon. So I'll go for the custom built PC after all.