I really can't recommend the GTX 660s because they have a gimped 192-bit memory bus -- Really, anything from the 6xx series is gimped in terms of what the 7xx series can do, but in order to move up to the 7xx series you're going to have to pay more.

The GTX 760 is probably a much better bet, and two of those would run you ~$500. The AMD R9 290 (non-X) is at ~$500 as well and it's got slightly better performance than a single GTX 760, but if you were to SLI those GTX 760s while gaming, then they're likely to come out as the top performer.

Here's a quick breakdown:

World of Warcraft
GTX 760
R9 290

But since TechPowerUp didn't do a GTX 760 SLI benchmark for WoW, we'll use a different game just to show how the two setups compare against each other in terms of horsepower:

Tomb Raider (DX11)
GTX 760 (x2 SLI)
R9 290

I just picked Tomb Raider for the hell of it because it was DX11, but you can look through the rest of those two Guru3D reviews if you want more of a comparison between the two different setups. Keep in mind that benchmarks can sometimes go either way when looking at certain benchmarks as some games tend to run better on nVidia hardware than AMD hardware, and vice versa.

I know you said you didn't want to run SLI (at least right now), but you can practically double your performance in games by doing so (or even split the load between the GPUs if necessary). I'll be honest though, if you're not accustomed to swapping in and out of SLI with so many monitors, it'll probably be annoying since you're likely to be bound to only two monitors when running SLI -- I'm not entirely sure which ports on the card are still active when running SLI, but I can run two monitors via DVI from the "main" GPU when running SLI with my 7xx cards. Upon disabling SLI you may have to readjust your monitor layout in Windows when they all come back online, but I've become so used to doing all these things it really doesn't bother me anymore because I get a ridiculous boost in performance with SLI (and I've done it so often that I'm damn good at it at this point).