I guess you are not aware of the new system. Dell encourages overclocking on this system it includes ntune and other software for oc. The power supply is 750W which is plenty. They specificaly made this system for easy ocing.
No personal experience, but it seems it's fairly setup for overclocking and all...
For folks that want to take performance a step further, you will have the opportunity to overclock the parts of the system through the system BIOS and software like NVIDIA's nTune application, much like the XPS 720 today.
I'd still go with a custom-built machine, though. You could probably get better results out of it. I could also see Dell setting some limits to the machines since it's fairly easy to over-overclock a machine