How to Justify getting a GTX-670 Just for WoW.

Well my current system is getting a tad long in the tooth (Core2Duo E6600, DDR2, GTX-260) and is running like a dog - sub 15fps in capital cities with detail settings set to absolute minimium.

Wondering how I can Justify to myself getting a GTX-670.
Seen one on special for $399 AU (most 670's are 430-500).

Anyone using a 670 for wow only?

If I am honest, you wouldn't see any more of a difference than if you were to spend 130 on an HD 6870 or 7770 due to the fact that you have that processor. However, if you intend to upgrade the rest in the near future (i.e. before christmas) then go ahead. Although I'D get an HD 7970 instead that is the same price but better (and you would have to spend 400 dollars less when you do buy the rest of the PC).

You would see your maximum FPS go up to 30-40 FPS, but no promises on your minimum FPS. A new processor (and motherboard and RAM) would push your minimum FPS to 15-30 fps

My setup (never goes below 30-40FPS, not even 25LFR at ultraxion or madness P5 at full ultra settings)
Intel i5 2500K (4.3GHz because I'm lazy and I can get that on stock vCore) | Asus Z77 Sabertooth | 8GB Corsair Vengeance LP 1600MHz | Gigabyte Windforcex3 HD 7950 | Crucial M4 128GB | Western Digital Caviar Blue 500GB | Noctua NH-U12P SE2 | Fractal Design Arc Midi | Corsair HX650

Given you have an E processor, I am guessing you have a socket 775 MB, which is what I am running on. I am running the Q8400 processor with a GTS450 video card and 4GB RAM, and I usually have a minimum of 30fps in cities. If you are not looking to upgrade your entire platform, that's not a bad CPU if you can find it. I am running at stock speed (2.66 GHz), but they overclock to 3 quite easily on stock cooling.

It can be quite tricky to find an LGA 775 processor nowadays, and given that the GTX 260 isn't a bad GPU, even nowadays I would tend to recommend upgrading to something like an Intel i3 or i5 with an AsRock Z77 Pro 3/Extreme4 and 8 GB RAM and using the GTX 260 until more funds become available.

i.e. something like this:

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: Intel Core i5-3570K 3.4GHz Quad-Core Processor ($234.00 @ PCCaseGear)
Motherboard: ASRock Z77 Pro3 ATX LGA1155 Motherboard ($115.00 @ PCCaseGear)
Memory: Corsair XMS3 8GB (2 x 4GB) DDR3-1600 Memory ($48.30 @ Mwave Australia)
Total: $397.30
(Prices include shipping and discounts when available.)
(Generated by PCPartPicker 2012-09-07 22:40 EST+1000)

"Wondering how I can Justify to myself getting a GTX-670."

You can't. Just get something more reasonable, then buy something reasonable again in 2-3 years which will be better than the 670. Spend less overall.

You're limitation in your current system has nothing to do with your video card. The 260 you have now is not limiting your WoW gameplay in the slightest (assuming you don't have a 30" monitor") The rest of your system is what is holding you back. You should be able to run near max settings with the 260 and a good processor and push good FPS (40+). You can't really expect huge frame rates when you are in the cities, there are alot of other factors that come into play there.

I added some new RAM to my system over the weekend - got a deal on 8 GB G.Skill Ripjaws for $55 NZD. The faster RAM allowed me to use my stupid motherboard preset to overclock by 15% (Asus p41 is not OC friendly), so I am running at 3.053 GHz on air, nice and stable per Prime95 testing.

I know the i3/i5/i7 architecture is much better, but it's all still quite expensive and I try to get around 3 years out of an upgrade (I'm in my 2nd year of my Q8400).