I have not used WinME for some time as tho it is not as bad as a lot of people say, it is much less convenient than either Win2000 or WinXP, particularly in dealing with media files and large HDs. I know that Win2000 and WinXP do not handle larger partitions (above 137GB) without having their Service Paks installed (SP4 and SP1 respectively). Be careful that your WinME is set up to handle your maximum partition choice as it is an 'older' system.
Also I think there is a way to force Windows to maintain letter identification to your pre-existing drives so that it will give your latest partitions the higher letters.

Good luck!