I have the same iMac as you, probably. Mine is the high-end 2012 iMac (i7 3.4Ghz, GTX 680MX).
I'm still jumping on the new 5K iMac. 4x the number of pixels is hard to say no to. It's ALL about the display, and I can't wait. I'm a total resolution junkie; I'm that guy who said no to an iPhone 6 and had to have the iPhone 6 Plus because of the number of pixels per inch.
If you're *that guy/girl* too, do it, do it, do it. I have a feeling when the impressions of Macrumors users come in, there's going to be an awful lot of gushing..
iMac 2012 - 2560x1440 = 3,686,400 pixels (109 pixels per inch)
iMac 5K 2014 - 5120x2880 = 14,745,600 pixels (218 pixels per inch).
If ever you doubt the decision, refer to the above numbers.
The iPad 3/4/Air/Air 2 have 264 pixels per inch. Now think how close you hold that iPad. Now look at how far away you sit from your iMac. Those iPads have 21% more pixels per inch than a 5K iMac, but... - again, think distance.
Personally? I think the new 5K iMac is going to look better than anything you've ever seen. Better than an iPhone 6 Plus, better than an iPad Air, better than a Galaxy Note 4 or LG G3, better than.. well.. you'll see.
I'm so glad we're finally in the age of 5K screens, and that Apple kept the real estate of the 27" iMac intact. That makes going from our 2560x1440 iMacs to the 5K iMacs a simple, simple change.
Conversely, going from a 15" MacBook Pro with the BTO 1680x1050 display to a 15" Retina display MacBook Pro was much harder, since you lost real estate in HiDPI mode (Apple used the base 1440x900 resolution and quad-pumped it to 2800x1800). That meant those of us who owned 1680x1050 non-Retina MacBook Pros actually lost space in native res when we bought 15" MacBook Pros with Retina displays. Boo!
Anyway, getting sidetracked. Can't wait for 5K iMac. Buy one. That is all.