How can that be possible? I'm hitting only 270mh with poclbm on both my 5830s clocked at 900 core, using Afterburner to overclock but anything past 900 (Enabling 'unofficial' overclocking in the config) puts the system in an endless BSOD loop.

Am using these for what it's worth: http://www.scan.co.uk/products/1gb-xfx-hd-5830-pci-e-21-%28x16%29-4500mhz-gddr5-gpu-840mhz-1120-cores-dp-2xdl-dvi-i-hdmi