In about a week I will be redirecting about half of our 250 pages to a different site. they are all internal pages (one to two clicks away from the home page). Most of those pages have little to no PR from external pages, and their PR basically comes from the site's home page.
Should I keep the Site Map links up that link to their current address (which will be redirected next week)?
Or should I remove the Site Map links for those pages that are going to be redirected?
Or, should I change those Site Map links to the new URLs?
Assuming all other things being equal then backlink power should be transferred, although Google said that little bit of juice is lost by doing 301
TBPR (Toolbar Page Rank) information you are viewing will disappear. It may come back the same on the next toolbar page rank update or it may not be the same. Last time Google needed almost 8 months to update TBPR so you may have to be prepared for a long wait to see what happens with TBPR. I would not worry about TBPR though.
Ranking may suffer for a short while impacting the traffic until Google starts ranking new page instead of the old one, my experience in general is that the new URL replaces old URL in SERPs after some time and traffic then recovers.
There are always risks, but the above has been my experience.
But you must do permanent 301 redirect and it must be in one hit.