Keep in mind the X-3 as of now has a high likelihood of coming as two separate physical units, in which case scaling up the power supply on a single physical box wouldn't require such high wattage single power units. A 750 watt unit might already be almost 50% over spec (assuming 1050 watts total / 525 a box).
If it is 2 units that could each have a PSU, that would be probably make things a lot more doable for overclocking.
Is this based on the drawings at Black Arrow? I've wondered about those -- they also seem to indicate that each of those 2 units is 2U -- thus for one total X-3 you might actually need 4U? I guess I'll post that question over on the black arrow thread just to clarify. Hopefully they're getting the specs tightened up enough to have a good idea of form factor and so forth. I sure hope that isn't the case as I just ordered a smallish rack to setup over the holidays.
I'm also looking at power conditioners and
these 2400W Tripp Lite ones seem to be pretty good -- one per two X-3s. They have a
rack mount version too, but it's about $60 more expensive and takes 3U. I think UPSs are too expensive for me, and my power isn't too bad. Just rather have it very clean for longevity.