Just install the driver, not CCC. Instead, install
ATI Tray Tools. Either can do the OC, but I much prefer to work with the latter.
(ATT works with CCC installed, so you could just install both and ignore CCC)
Cards come with reference clocks; the clockspeeds determined as normal by the chip manufacturer (Nvidia, ATI, etc). Any card that deviates is overclocked by the board manufacturer (XFX, Asus, etc). Such cards are tested and confirmed to work reliably on those speeds, though you can usually take a reference model and do the OC yourself, but you have no guarantee it will manage it then. Might void warranty in that case as well.
Can't you get the 280 replaced under warranty?
*edit: I mean, why go for an ATI, that you say you'd rather not get, that's only marginally faster than your 280? If it's still under warranty, you'll probably get a 285 as replacement and you're good to go for another 2 years.