I am surprised that no one has mentioned this. To do straight lines, what you need to do is click somewhere that is your starting point. Then, hold shift and click the second point. What this does is 'connect the dots,' effectively creating straight lines in any direction easily and quickly.
As for digital tablets, two things I see people overlook are pressure sensitivity options and weight of the product. The Cintiq and Intuos4 have pressure sensitivity of 2048 levels. The Bamboo has 1024 levels (same as Intuos3). I have been painting with 1024 levels for years and that has served me well.
What I don't recommend is to get a Cintiq because of two potential reasons: heat from the screen and portability issues (bringing a powered screen everywhere really limits your drawing time). A Cintiq is good for architects and photographers...since they are on the move, but horrible for artists that are serious about doing art as they will be drawing for hours on end.