Really it's up to you. personally i prefer to use the pen tool, but if you want to finger-paint, you'll need to do a bit of research on the matter (shouldn't be too hard to find)
Intuous It Is!
Good choice, the bamboo series really aren't that great.
I have an intuos small, works just fine.
Sai actually does support pressure sensitivity, plus it has a lag time feature that comes in handy for vector layers.
For drawing, it's just as good as photoshop, and a lot smaller.
For editing, well photoshop outclasses pretty much everything.
For a free photoshop knockoff, for the editing aspect, you can use gimp.. But gimp is quite large.