I am also very interested in this issue. I have a 6GB Sansa e270 and am carefully rating songs from my 7K song collection to create smart playlists based on certain factors (shown below) and I am not quite to the limit but I am sure I will have trouble coming up when I go to sync. I am upgrading to this player from a 128MB player I got for like $40 a while back and had many problems synchronizing since it is so small to start with!
This is a great configuration line for a playlist that others may want to consider using. I have been tweaking it as I see fit but this is the latest and greatest, version 2.2 (not joking about version):
([Rating]=>=3 or [Number Plays]=>5) -[Genre]=[Children],[Christmas],[Classical],[Comedy],[Halloween],[Meditation],[Nature],[Poetry] ~nodup=[Artist],[Name] ~sort=[Rating]-d,[Number Plays]-d
It does it all...from picking intelligently based on rating and number of plays, including sorting by the best option for this. Now I just need to see if I can add an IF statement so I could make it do even more ideal things. But this removes genres I definitely do not want (I could add Christmas around the season and automatically get my favorite christmas songs added). It finally checks for duplicates based on artist and name because I have many greatest hits albums which overlap with other CDs.