I've noticed a little more ram usage on the server side of things. Although, I will NOT make the assumption that this is at all related to the FTB pack. FTB is running 1.4.2 (1.4.5 if using the magic pack) while tekkit is running 1.2.5.
Lots of changes were made between those versions and it could really be anything that's causing my server to use a little more ram.