Its very fast and a worthwhile upgrade imo as it wouldn't cost too much. As for the GTX 280, the 9800GX2 has spoilt its party and its far too expensive for what you get. Everyone seems to have forgotten the the 260 is a fantastic card in its own right.
Also looking at firingsquads numbers, an overclocked GTX 260 is almost as fast as a stock GTX 280. For $250 less its a bargain.
Core i5 2500K@ 4.5, KFA2 GTX 570, Gigabyte Z68, 8GB Corsair XMS 1600MHZ, Antec Truepower New 650w, Coolermaster Storm Sniper, 1TB Samsung F1, Yamakasi Catleap 27", Samsung 245B, Asus Xonar DG...blah blah blah