That's because you have all the eye-candy turned up (I'm not talking about AA/AF), much of which makes a very small change in image quality.
As was mentioned by psolord, this is highly subjective. I can immediately tell the difference between medium and high settings on every game in my collection. I can also immediately tell a difference with and without AA, especially on transparent textures.
A 4870 is way more powerful than the XBox or PS3, for example, and the latter two provide 90% of the non-input-related gaming experience that you get on a PC.
True, and then, false. Your perception of the experience may be very similar, and you are certainly entitled to your opinion on that matter. But it is
not "90%" of the experience to me -- especially in terms of overall "eye candy" that you have so easily dismissed. All of that 'eye candy' that you think doesn't matter? That's why I use my PC to game.
However, everything is subjective, and you may really enjoy the incremental details that a 570 SLI provides.
A 570SLI enables far more than "incremental" details over a 4870 in both an absolute and relative sense. Far more texturing, far more antialiasing, far more pixels on your screen, far more frames per second on newer and more demanding games.
I personally have moved from an r9700 (unlocked 9500np), X800XTPro, 7950GT, dual 3850's, dual 4850's, a single 5850 and now to a 7970. Every upgrade was precipitated by the fact that I couldn't run the games I want to play at the resolution and 'eye candy' settings that I wanted to. Far Cry with all settings at full at 1600x1200 on a Radeon 9700 with 4xAA? Nope, not happening. Didn't work on an X800 either, but was mostly attainable on my 7950GT.
Dont get me wrong - my overvolted, hacked-firmware overclocked 5850 was a fantastic performer for it's age. But even with my Dell 2007WFP 1680x1050 at 4xMSAA it struggled; and we're not even going to talk about my new Dell U2711 2560x1440 beast at 4xSSAA. It would've been a slide-show. Even my overclocked 7970 can't sustain 60FPS in the most demanding scenes with this configuration.
And that's the kind of configuration that I
expect to play with my PC, and that I'll not be seeing with any console for another few generations (at the earliest.)