To overclock the GTX 580 we chose to use NVIDIAs own System Tools software which adds a performance section to the driver. In this performance section we can change the core, memory and shader speed of the card.
Our chosen settings were based on Point of Views TGT Ultra Charged card which is available with speeds of 841MHz core, 1682MHz shader and 4104MHz memory.
For the Radeon 6870 (in CrossFire) our overclocking was performed using AMDs Overdrive Software which again allows us to configure the speeds of the card within the driver. For this set of cards we opted to configure them at the level of HIS 6870 Turbo which uses the standard PCB and cooler found on the reference cards. HIS use 920MHz for the core and 1120MHz memory.
Finally we have the 5970, a reference model with 725MHz core, 1000MHz memory.
In the UK these cards retail for £430 (580OC) and £460(6870CF Turbo/5970) at Overclockers and SCAN. In the US Newegg has overclocked 580s for $579, HIS 6870 Turbos for $275 ($550) and the 5970 sits at $499.
NOTE: All of the above are fluctuating on a daily basis.