I can offer my opinion. I am a photographer/videographer. I use Final Cut Studio and Adobe CS5 suite. Sounds like our needs are very similar. I was comparing GeekBench results today to try to answer the same kind of questions (I know not by any means an inclusive answer but hopefully helpful data). My 2006 Mac Pro has an ATI Radeon 4870 graphics card, 4 GB of RAM, and I have about 1.5 TB of hard drive space. The GeekBench for my system came in around 5500. The average for the 8 core current Mac Pro is 11,735, whereas the current Quad Core Mac Pro is 8216. The score for the i5 (might be the i7) is 8237. I also use a an older 2.16 Core 2 Due 20" iMac which scores 2842.
I think the old iMac (2842 score) is capable and my Mac Pro (5500) is comfortable. By that I mean I kind of wait from time to time with the iMac but only wait for the Mac Pro when I'm doing a final render or video coding. I can use Motion quite fluidly and the actual editing of video is mostly in real time. Aperture hogs Memory and the graphics card but processors usually aren't strained and Photoshop is usually pretty instantaneous. In my opinion if the computer is waiting for me it is fast enough, anything more than that is overkill not being utilized. So basically with my current workload I want something as fast as my Mac Pro.
So in short I've decided that the iMac and the Mac Pro (current models) are both luxury (more than what I'm comfortable with and slightly overkill that will be used with really intense things from time to time). The question comes down to do I want things internal or external - like hard drives, blu-ray drives, etc. and how much do I want to spend.
I'm definitely waiting for a refresh in hopes that a 2 TB drive option is cheaper, that the overall configuration is cheaper, and the graphics card update to the 5800 line. It's not really about the processor now as it is the video card and amount of RAM.
My dream configuration is a 27" iMac i5 (or i7) with 4-8 GB RAM stock and another 4-8 GB RAM 3rd party, 2 TB HD, ATI Radeon 5870 graphics card for about $2,500 ($2,000 would be nice).
Given what the current iMacs are configured as I don't think this is out of the realm of possibility. The big thing that keeps me away from the Mac Pro is for a similar configuration I calculate it will be somewhere between $1,200 and $2,200 more depending on if you get the 24" display or the 30" display. It is just way more money.
So answering your questions:
1 - I think the 27" with a graphics update is good enough.
2 - I think the quad core is worth it.
3 - I'd wait for an update.
4 - I doubt prices will be more - I'd guess about the same or maybe a bit less.
A long winded answer but hopefully it helps you in your decision.