OK, Whoops. Sorry again for the long post.
Yeah, these things do tend to end up being Mac vs. the rest. In this case however, I think that the iMac is a beautiful thing but very overpriced and somewhat ill suited for photo/video work. Yes, it CAN be used for those functions but it's very expensive and inefficient to create a good environment for the task. IMO, when working with thousands of important images or large video files, the important task (that shouldn't be overlooked) is not just CPU or RAM, it's file management. The images should be on a RAID volume to offer protection from failure and then also stored on a large secondary volume for backup. An iMac is essentially a laptop with a big screen and isn't designed to provide this type of multi-volume fault tolerant infrastructure. (Without hanging a bunch of expensive Thunderbolt drives off the back, which is still very new technology.) Yes, you can do fine with an i5 CPU and 8G of RAM (although OSX has always been very RAM hungry) but you can't do anything if your data or system is lost.
Whether the OP buys Apple or Wintel, the advice should point them to something that is powerful, configurable and upgradable. I would suggest a Mac Pro, not an iMac. I don't think the Mac Pro is a rip-off if that is what the job demands but it is still very expensive, just like all Apple products. I didn't set the prices. The Mac Pro disk structure, CPU and RAM can be much further expanded and is better designed for what the OP is doing. (Esp if they venture into video!) The iMac isn't very expandable. It's primary purpose is to sit on a desk and look pretty and provide a nice device for desktop activities like web/email/iTunes and basic home duties.
Please remember that Apple products are built first and foremost to steer Apple users into the Apple stores to consume and purchase media/app content and other Apple products. Getting other functionality from them is a secondary priority. The iMac is a perfect example of this. It is marketed to the affluent crowd that already owns iPads, iPhones and iTouch devices and want to venture into an Apple computer. Most long time Apple owners that I know or read about purchase MacBook Pros with Retina and MacBook Airs these days. Those are the most bang for the buck at this time. The iMac is the least bang for the buck. Production houses and graphics firms either use Mac Pros or they have started investing in comparable WinTel boxes.
Bottom Line is that if someone is looking to spend $1500 -$2000 on a device, I am going to help them spend that money wisely on the best choice, not just get what looks nice and be good enough for the time being. Unfortunately, an ideal appropriate Apple device doesn't exist for photography/video work for less than over $2000. That's just the way it is. Again, I didn't set the prices. Which is why I have an exceptional WinTel box (comparable to a Mac Pro) instead for literally thousands less. I don't do my serious photo editing on a laptop (regardless of the make) and I'm not using a limited All-In-One Monitor+Laptop Hybrid machine either. It doesn't matter whether it's Apple or not. The tool should still be appropriate for the task. If you've ever helped/consoled someone who lost everything due to a drive crash, you start to realize the value of fault tolerance and easy automated backups. If you want a great WinTel laptop device for photo editing, check out the Lenovo ThinkPad W530. It has features designed for photography work like built-in color calibration, an IPS matte display and a built-in RAID disk option. For less than a comparable Apple laptop. But it's not brushed aluminum so there's that to consider.
I would suggest either getting a MacBook Pro with SSD-Hybrid drive + big external disk for backup and then an external display or a Mac Pro with RAID and internal backup drive + external backup drive with external display. The iMac would be my last choice for photography or video editing. It's just too limited, crippled and expensive. At least with a MacBook there would be the added convenience of portability, a better screen and an i7 CPU.
Trouble is...Aperture and Final Cut Pro X don't tend to work too terribly wall on that Wintel box you're talking about.
And, if someone is needing tools lessor than that for real amateur work or just getting to learn things, OSX which comes with a mac, has iPhoto and iMovie included with the purchase.
If you go the Windows route, sure you can spec a box that is a bit lower in price to the mac (mac really had no low end machines)...but you also have to spec in the cost to buy all of the software that Windows does not come with, and that adds price to the bill.
Me? I figure whatever tool for the job. I have linux boxes I like to play with and run as my servers at home. My current main work computer, is a loaded up macbook pro, late 2011 model. I loaded it with 16GB ram of my own, i7 cpu, largest disk they had..etc.
I use that as my basic work station. I run Win7 (I won't touch Win8 till I can't help it) in VMWare on it for my windows needs. I have VMs for different distros of Linux when I want to play with that...
I have it hooked to a Dell U2711 IPS monitor, I run keyboards and mice hanging off the monitor's USB...so, basically my macbook pro is a desktop at my desk, but I can travel with it too, just pick it up and go.
I'm about to finish up and get a freeNAS system on my network for massive storage, all using ZFS to basically supplant RAID...and it will allow everything on the network to access and back up to it.
So, anyway....any tool for the job.
I like things about the mac. I cut my teeth with iMovie and iPhoto...and for a very small price, have upgraded (only $30 for Apeture and only $300) for FCPX....MUCH less than what I'd have to pay to get the full Adobe Suite (although I am working a deal to try to get an educational discount for the suite which would only be about $450).....
So, if you add up all the software with comparable hardware on the Win vs Mac system....it gets much closer in absolute dollars spent.