The 680MX will be similar to a desktop 660
While it has similar specs to a full GTX 680, the clock speed is around half or so, along with the fact its a MOBILE GPU, which are never as good as they sound/look/seem based on specs. For example, the 660m is around equal to a desktop GT640 (GDDR5 version, not ddr3). The 650m in the base iMac and MBP is also not equal to a desktop 650 (which is VERY low end already), and I'd would say(guess) its similar to a desktop GT 640 DDR3.
Apple will also most likely under clock it further, due to the fact that the new iMac has HORRIBLE ventilation, and is inadequate to cool a card of this power at stock speeds. I would imagine the end perf. is between a desktop 7870/660 and the 7950/660ti (closer to 660).
Also, calling a Mac a gaming machine is an incorrect statement for a few reasons. A) 99% of them have mid-range hardware (the exception being the Mac Pro's CPU[ultra high end except clock speeds] and the 27in iMac 680mx[not high end, more like mid-high end]) and would be inadequate for modern, high end games. B) Macs CANT be a gaming machine as they don't have games to play(lol).
Another point, no mac game will require anything more than the 660m in the base 27 in, or at the most, the 675MX will max any mac game.
Basically, don't pay for the 680mx or 675mx unless you need it for rendering/3d work (at which point a Pro is better) and buy a lower end model. Use the saved 800-1000 dollars to build a PC with equivalent graphics if you wanna game (or access windows only apps) and have the mac for everything else.
tl;dr its not equal to a GTX 680 and is closer to a 660ti, don't waste your money. Use the saved money for a PC for just games, you'll be happier that way.
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Real men game on Falcon Northwest computers such as this one I configured for myself. Note the price tag at the bottom but this is the real deal, no compromises. Unfortunately, it does not run OS X so you will of course need to purchase an appropriate Mac for your needs as well.
Here is your customized Mach V system configuration.
System Details
Chassis
ICON2 Exotix - Any Single Color
Chassis Logo Insert
White Light
Chassis Fan Kit
Performance Fan Pack
Sound Dampening
AcoustiPack Sound Dampening Foam
Power Supply
1500 Watt Modular
Motherboard
Rampage IV Extreme
Processor
Intel® Core i7 3960X 3.3GHz
Processor Cooler
Liquid Cooling - Mach V
Processor Overclock
No Processor Overclock
Memory
Elite 1866MHz 64GB (8x8GB)
Video Card
Quadro 6000 (6GB)
Video Card 2
Quadro 6000 (6GB)
Video Card 3
Quadro 6000 (6GB)
Monitor
30" 2560x1600
Sound Card
On-Board Audio
Speakers
Z-523 2.1
Networking
On-Board Ethernet
Hard Drive
m4 SSD - 512GB
Hard Drive 2
m4 SSD - 512GB
Optical Drive
24x DVD Writer
64-Bit Operating System
Windows 8 Standard
Office Software
Office Professional 2010
Warranty
3 Year Warranty - Mach V
Shipping options
UPS Ground : $240.10
UPS 2-Day : $624.03
UPS Overnight : $966.24
USPS (APO Only) : $915.00
UPS 3 DAY : $366.00
Total System Price: $17140.75
This my friends is what a top of the line gaming PC costs not to be confused with something cobbled together with parts from newegg.com. This bad boy is a state of the art PC gaming rig.
Personally, I think the iMac is a better value given that it can also run games pretty well for most mere mortals and costs quite a bit less plus you don't need to buy another computer just to have a decent desktop operating system (OS X).
you sir, do not know gaming desktops, that system is a piece of overpriced garbage. all desktops use the same parts.....and new egg is the best, cheapest way to buy them INDIVIDUALLY. i could build a pc from new egg at half that price that would still beat it handily.
also, quadro 6000 isn't for gaming, and a 300$ desktop GPU would beat it for 3d gaming,
if i wanted i could double gaming/productivity performance for the same price if i chose parts from newegg (think 16 core, 32 thread. 4x7970 ghz[it beats 680 now] 96 gb 2800 RAM plus some sweet watercooling)
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44.8fps on ultra so yes pretty much.
bootcamp will slow it down a little, so paying this much for 40fps at nonnative 1080p is a rip
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Are there even any big benefits for the normal home user to get the CPU upgrade for the 27"? Would it extend the lifetime of the iMac?
nothing really, an i7 is NO diff from i5, except HT
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Nvidia "wants" it to be at the speed it is running at - there's a reason it's a clocked down desktop part; heat, power consumption and noise.
Sure, you might be able to push it up to desktop speeds, but without knowing what the thermal overhead is, or even if the iMac can push the necessary current to the card in the first place if it's overclocked by 30% (the supposed delta between the desktop and mobile version) you might have to deal with instability etc.
apple doesn't support drivers with the ability to over clock.

the necessary nvidia based drivers are PC only
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my opinion: do what I do, have a mac for general use, and then a high end PC for Games, rendering, heavy power use
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The 680M is currently the fastest mobile GPU there is. It's just slightly slower than the desktop 670. The 680MX, which is not in any device right now and appeared on Nvidia's product pages this morning is even faster than the 680M.
Your 5750M is very far behind and many times slower.
680m is slightly better than a GTX 660, the 670 would blow it out of the water
to clarify, for nvidia: GTC 690>590>680>670>660ti>580>680mx>660>680m