For those who think their overclocked GTX 280 is still too slow, VR-Zone got the right thing: A Volt mod manual.
Volt mod for the Geforce GTX 280: the braze points (picture: vr-zone)
The first of the two pictures specifies the location where you can tap the real voltage of the GPU - this is essential for stability. The second picture shows at which points a 100 Ohm resistor has to be brazed onto the card, in order to permanently increase the voltage of the Geforce GTX 280.
Unfortunately there aren't any temperatures or clock speeds given, which result from this modification.