my one 15" monitor supported 720p wonderfully, but my 19" MAG monitor messes with the top inch of the 720p picture (it smears it accross the screen). its playable, but not worth it.
I think your best bet might be to ask bestbuy or someone if you can try your xbox on their display units to test if they work well (although they might not like the whole modded thing).
oh, one other thing, is that if you plug in a monitor then turn the xbox on, it might not work, but if you turn the xbox on, then plug in the monitor it might. just a thing to try if you think your monitor isn't working. this got many lcd models to work for me.