Another am I ordering the right iMac question.

Unless you really plan on getting into video editing the 27" i5 is fine. I love the 27" screen and the extra screen real estate is nice when you move up from 24"

Yup. Sounds perfect to me once you upgrade the RAM.

I specced the i5, Fusion Drive 1TB, 680MX and 8GB RAM (to be upgraded with Crucial Ram to 32GB) for simlar usage i.e.

Lightroom 4
PS CS6
Occasional gaming (but reasonably demanding games)
Internet browsing
iTunes
Main computer for my iOS devices etc

Excited as by all accounts it should be more than adequate for my needs

I certainly expect a long life from your new 27inch.
I can tell you, owning an iMac really "redefines" the desktop experience.
Treat yourself, spend about $2000

The i5 and i7 are both quad core CPUs so both can process four threads simultaneously. The biggest difference is that hyperthreading is enabled on the i7 which allows four additional logical cores for those applications designed to take advantage of this feature. Applications that can use more than four cores are relatively sparse. Primarily, we would be talking about applications for encoding video. If you do a lot of work with editing and encoding video, then the available benchmarks suggest the i7 would be a good choice. If that's not something you do a lot of then you are unlikely to see any benefit from the hyperthreading feature. For most other applications the i5s and i7s running at the same clock speed benchmark nearly identically.

Of course, the selctions for the iMac are not identical in clock speed so this is another consideration. The i7 is clocked 6% higher than the i5 and can clock up to 8% higher when turbo boost for each CPU is compared. However, 6-8% higher clockspeed does not necessarily mean it benchmarks 6-8% faster. For many uses, a 6-8% difference in clock speed will result in only very small difference in benchmarks. So if you don't use hyperthreading video encoding apps very often, it's probably not a good deal to pay $200 for a very small bump in clockspeed. It's unlikely you will be able to discern the difference in any real world task.

In regard to the GPU, the differences really come down to gaming. Both GPUs are going to perform about the same for nearly all other functions. There really aren't a lot of good benchmarks available yet for the 680MX. The specs are certainly impressive and would suggest a mobile GPU that should perform nearly as well as some mainstream desktop GPUs. The 675MX has respectable benchmark scores for a mobile GPU and should play most modern games okay at fairly high settings. If you're only an occasional, casual gamer, you're probably okay with the 675MX. However, the 680MX seems to have the potential to finally make the iMac a serious gaming machine. If you're into gaming and care about eye-candy, then the 680MX would be a good choice.

I also think the fusion drive upgrade is a very good choice if a bit over priced. This is one upgrade that everyone should see significant performace improvements from in nearly everything they do. Regardless of your use scenario, you should get much snappier performance with an SSD onboard.

I couldn't think of any good reason for me to spend $200 on an i7 as I am unlikely to ever see any real world benefits from it in my personal usage. I did, however, spring for the 680MX as I've been building gaming PCs since about 1993 and can't quite give up the gaming bug now that I'm switching to Mac. I also felt like the fusion drive was a no-brainer for me.

Hope that helps.

I have a Nex-7 also but only do stills. I use iPhoto for photo management and simple editing and Photoshop Elements for more complex editing when required.

Running Parallels and Windows apps will eat up some memory. I only do a few things in Windows but notice the performance hit. 8 gb is the minimum and 16 will be great. My experience is that I use 3 to 5 gb when running only Mac apps.

If your employer has a Microsoft Enterprise license, check into whether you can get a Home Use Program (HUP) copy of Office. Gets you the Pro version for $10. You can also get both the Windows and Mac versions. I need the Win versions for Access - I maintain a VB app for a non-profit that uses Access.

My last iMac purchases was with the i7 CPU.

I didn't need it, but figured somewhere down the road if I decided to sell, it would help in resale value.

Hopefully I will at some point migrate some of my video work over to the newer machine so I can really exercise the thing!