Not likely to be the power supply. Oc'ing to 2GHz adds all of 2W to the power consumption. I have the IONITX-A-U and the only OC options for that board is FSB: 533 or 667 which makes the whole thing quite easy. I do have Kingston 2x2GB DDR2 800 RAM installed, though, which may be a factor. Try running the RAM unlinked.
Just a question, are you by any chance running your board fanless? It might also be an overheating issue. Adequately cooled, oc'ing should add no more than a couple of degrees but running fanless, it might get pretty dicey.
It's starting to look like I'm having an issue with both the RAM and cooling.
I've manually adjusted the CPU voltage at +.0325v and bumped the FSB to 600 (pushing the Atom a bit over 1.80 ghz) and it blue screens after about 20 mins of YouTube HD vids. I touch the passive heatsink on Atom and she's hotter than I ever imagined an Atom can get. Currently stable at FSB 590 with the DDR2-667 RAM unlinked.
I will pick up some two new sticks of DDR2-800 RAM and see if that takes some of the load off of the Atom. If that doesn't get her stable at 1.85 then I suppose I can apply better thermal paste and/or glue a 40mm fan on top of it.. Thanks folks!!