They'll probably look decent, though I doubt the iPad will upscale lower-res content. I wish iTunes would encode widescreen music videos the same way it does movies though. 640x360 is starting to look a bit dated.
This kind of doesn't relate but DONT GET ME STARTED... UGH..I have about 200 music videos and on my old laptop they looked okay but on my 27 i5 THEY LOOK TERRIBLE... Why are MV such low quality??!! It makes me SO MADE. I mean the LITERALLY look like **** on the 27. I'm talking pre-2008 youtube quality...(maybe not that bad but STILL!