1. Yes it is possible.
you can transition the active directory to windows server 2008. In place upgrade to windows 2008 is however not possible. You can add a new windows 2008 member server to the windows 2000 domain. you will have to prepare the AD on windows 2000 and then run DCpromo on windows 2008 ( install ADDS role first). once the windows 2008 is an additional DC you can transfer the FSMO and GC roles and decomission windows 2000 server as a DC.
2. windows 2000 cals for TS are not compatible if the TS is a windows 2008. You can install Terminal services & TS lic service on the windows 2008. the license server can act as a valid LS even for legacy TS.
3. The AD objects if i understand your question correctly will be replicated to the new DC ( win 2k8)
some ref:
http://technet.microsoft.com/en-us/library/cc753437(WS.10).aspx
