Is there a reason you're choosing a compact computing format rather than a proper workstation?
My issue with iMacs is that they are basically using laptop technology in order to fit everything into that form factor. If you get one, you are going to sacrifice performance for the sake of beauty. They're really good-looking for playing back final renders for clients to see and they work pretty well for Photoshop and web design, but I wouldn't want them for intensive things like 3d rendering, After Effects or encoding video. If you don't want to wait for the new lineup of Mac Pros, I'd suggest getting a workstation with Windows 7. Windows 7 is a rock solid operating system and the Creative Suite runs really well on it. You can also get a lot more processing power for your money in a Windows machine vs. an iMac.
If you really want to use an iMac then buy as much memory as you can afford and a good external thunderbolt hard drive. That's about all you can do.
My experience is that over the long haul a Mac workstation is more ecconomical than a windows machine because if you really match feature for feature and hardware for hardware they both cost about the same unless you build your own. That's based on the last 8 years of running Mac's and PC's and counting all of the costs involved for both types of systems including the cost of maintenance and service agreements and IT time required to keep the systems running well. The biggest problem that I see with the new MacPro's is that they are not out yet and no one knows how they are going to perform or how they are going to hold up. It's been a long time since there has been a major improvement to the MacPro line and the big aluminum boxes have fallen behind similar windows machines from major manufacturers for quite a while.
I've got a first generation intel iMac that I bought at Mac World in 2006, hauled around like a laptop in an iLugger case for 5 years, and it's still running CS5 Master Collection and working every day. Total maintenance for the last seven years includes OS upgrades totaling about $200 and two new hard drives. I replaced the internal hard drive every 3 years as preventative maintenance. It's my sincere hope that the new MacPro's will have the same kind of service life but for now we'll all just have to wait. The longest that I've ever had a windows based machine run without replacing a major component is about 20 months. The longest I've ever kept one in service is 3 years.
My experience is completely different. Being responsible for all costs, I can't imagine how a machine, which is 30-50% cheaper for exactly the same performance, much friendlier for customisation and cheaper for an upgrade, can cost more in the long run, while all the components are manufactured in the same country. Sometimes at the same factory.
My DEC HiNote VP, which I bought in 1995, is still running and in good conditions (aside from being completely obsolete).
If you really need some advice from a hardware guru, you can contact e.g. Eric Bowen from ADK. He is quite active at Hardware Forum and can tell you a lot about PC vs. Mac performance 'Adobe products'-wise. Or Bill Gehrke, who was running Premiere Pro Benchmark website together with Harm Millaard.
Well, I was a Windows user from version 3.0 up until 2008 when I switched to a Mac. I found that from Win 95 forward, I was complaining about the same issues in my computing experience, and all were inseperable from the Windows platform. I say that simply to say that I've gone the Windows route before, and really have no desire to go back.
Why an iMac? I'm concerned that doing a Mac Pro is going to run me a LOT more, after everything. My main thing for the last 17+ years has been web design, and I have done a little video here and there. I want to push more into video as web content for my clients, but have not begun yet - so a large expenditure for a killer workstation would be pure speculation, and not based on actual work in hand. Basically, it's time to replace my existing machine, and I was looking at purchasing one at a slightly higher tier than last time, not a hugely higher one.
Also - as I noted, the goal is content for the web - shooting and cutting pretty simple marketing videos (talking heads, lower thirds, and lots of B-roll) and some animated infographic videos (I aspire to do things like this as my abilities mature: https://vimeo.com/album/1770463/video/25123730). I'm not planning on doing heavy 3D, or intense Broadcast AE VFX work that would require extreme rendering muscle (I don't think).
As to the Mac Pros - I have not looked into them very much. My concern is that they will be quite expensive (What did/do the most recent Mac Pros run? They are all gone from Apple's site). I also don't want to kill a fly with a bazooka. But I'm just not sure exactly how much power I'll need, and how much those deliver. If I was loading up my schedule with a ton of the kind of work that I knew was going to tie my machine up with renders, then I think a Pro would make sense. But if my kind of work is really not anywhere near that heavy, then, it would be largely wasted, no?
These days, Imacs are fine for video editing/compositing, especially for what you're doing. Their biggest downfall is the hardeware upgrade path down the line.
That said, I'd, max everything you can. AE uses the graphics card to accelerate/enable things. Ram is always good with AE and obviously faster processers... nuff said.
I don't think I'd hang around and wait for the new Mac Pros. who knows what they'll cost? The current lineup is overpriced for what it is.
If you've built computers in the past, the other option is... Hackintosh. Buy the correct parts and naysayers naysaying is simply the sound of a misuguided breeze in the distance.
In my research, there are two things which make a ton of difference. RAM and drive read/write speed. I try to max out the RAM on any maching I intend to use for AE and/or Photoshop work. But... I was bloen away by the speed gains I got when I added an SSD and assigned my cache rendering to that drive. SSDs are incedibly fast in rendering to and reading from. It's a great return on investment. Hope this helps.