What's the best specifications for 27" iMac in order to run Windows for best performance?

Question

Hey there. I'm based in the UK and in the process of saving up for my first ever iMac (the new 27" model starting from £1699) after years of owning PC's and laptops, but now I'm finally fed up with Windows-based machines, they just don't cut it when it comes to performance and workload...etc.
I wanted to pick iMac users on the community to get clear advice on what the ideal specifications I should consider configuring my iMac when I finally get round to placing my order in March/April. I aim to run Windows 7/ or 8 via Parallels as I hear it's very good and from what video tutorials of it I've seen on YouTube it appears impressive to think the iMac can handle this at the same time as running Mountain Lion.
The ram I plan on upgrading myself manually, yes - I've sussed that one that very quickly due to Apple's extensive £400 fee to simply upgrade the iMac to 32GB, Crucial here I come!
...I digress a little. I'm torn between the Quad-core i5 and i7 as I don't want to get something that won't deliver best performance when it comes to running Windows in parallel (or coherence mode as I hear it's called). Below are some specs I've configured on 2 saved iMacs in my saved baskets.
CONFIGURATION #1
3.2GHz Quad-core Intel Core i5, Turbo Boost up to 3.6GHz
8GB 1600MHz DDR3 SDRAM - 2x4GB
1TB Fusion Drive
NVIDIA GeForce GTX 680MX 2GB GDDR5
Order total: £2,019
CONFIGURATION #2
3.4GHz Quad-core Intel Core i7, Turbo Boost up to 3.9GHz
8GB 1600MHz DDR3 SDRAM - 2x4GB
3TB Fusion Drive
NVIDIA GeForce GTX 680MX 2GB GDDR5
Order total: £2,299
I'd appreciate anybody's time and patience on this little matter. Thank you.

Answer 1

You don't need to go overboard on your hardware to run windows effectively.  I bootcamp win7 on a late 2008 MacBook Pro with no issues. However, for an intense windows game I boot straight into windows.  If you do not plan on gaming other than an occasional instance by your brother than this should be an acceptable solution. 
One caveat is ram which you have already stated your are upgrading. 
A general rule of thumb I live by is get the best you can afford now and it will prolong the need to upgrade in the future.  That being said...  The choices you have provided are all good choices.  Preference on screen size will dictate your overall decision, of course.   The need for i5 vs i7 CPU is normally not a factor for the majority of users.  If you need to save some money then by all means choose the lesser configuration as it will handle windows fine.
One place you should consider the upgrade is the video card.  Since you cannot upgrade this after purchase it is advisable to do this now.  Also, the differences between the 675mx and the 680mx are far greater than what the naming implies. 
Also, as mentioned in a previous post, the 3TB fusion drive can't bootcamp windows.  There is a hack for the non-fusion version, but I have not heard of anyone getting that hack to work on the fusion drive.   There are other creative options you can implement to make this work, however, it will require an additional investment in some 3rd party hardware.  For a novice Mac user I would recommend the smaller fusion drive and external thunderbolt/USB storage as dictated by your needs in the future.
HTH