I suggest that you do it after you've set up your system on the new motherboard. Once the new board is settled in with Win98SE (or vice versa) you can set about collecting Win2K drivers for all your hardware devices. Do this before you upgrade, and have the drivers sitting in a special folder - ready for when Win2K starts firing off requests for them.
With regard to drive imaging, I don't know of a good free utility. I always use PowerQuest's DriveImage 7, which at 69.00 euros isn't prohibitively expensive, considering the important job it does.
click here and take a look for yourself.