The GTX 280 is most useful @ 2560x or if you want to pile 16x AA on the games you play @ lower res. An overclocked GTX 260 offers a lot of stiff competition to a GTX 280 @ 1920x and lower. The same may be said for the HD 4870. The 4870 sometimes falls down in min frames and that will equal to a rougher gaming experiences. You'll feel a drop to 9 much more than you will one to 20-22.
If you're lower than 2560x and you think the GTX 280 price is a bit much for you, try a GTX 260 from a company that supports user overclocking without voiding your warranty, like Evga or XFX.
So good reasons to have an overclocked GTX 260 or 280 besides solid average frames might be; no worries with multi GPU stutter or bugs, and solid min frames. Hard crashes to FPS below 20 you will definitely feel in game play.