TTBOMK there's no inherent difference in visual quality between Fermi and Kepler. The extra memory on the Kepler might perhaps let you get away with 'forcing' a more memory-intensive form of anti-aliasing like Sparse-Grid or Transparency SSAA, but aside from that ... you should be looking at the same image. Maybe occasionally even a little better on the 2x480's since the extra umph might allow for a higher setting or two, as long as they don't cause memory usage to get too high.
Honestly, 480's are not 'cheap' cards. They were top of the line $500 cards just a couple years back, and at $200 they're a pretty crazy-good deal. Nothing else available can really touch their perf at that price. If you can tolerate the heat/noise, they're by far the best bang/buck cards available ATM.
I got rid of my very fast, heavily overclocked 470 SLI setup to grab a 670 mainly because that card came out at the beginning of the summer, and I live in Arizona. I'd lived through two summers of SLI Fermi cards already, and I wasn't looking forward to another one of them. My computer room got hot enough as it was.
If you want to see a fairly extensive comparison I did between my old and new setups (along with stuff like temps and power usage, etc), take a look at this thread.
My OC'd 470's were actually faster than stock 480's, and not a whole lot slower than 480's at their top clocks. So this will give ya good idea of what you should expect afa 480 SLI vs a 670.