^^
Wii Fit could, certainly moreso than being sedentary. Even actively doing something on a daily basis can have a more positive effect on your body than sitting down doing nothing. While Wii Fit would do absolutely nothing for me (I workout, running 3+ miles every other day with other kinds of muscular workouts), but if I saw someone who wanted to get into working out, it is not a bad tool.
BTW, for the OP, if you do get another console, I would opt for the PS3 for one of the two reasons:
1) You like Sony's first party content or/
2) You love movies and thus Blu-Ray is a must
If neither one of them serves you, you are better off getting an Xbox 360 which happens to be $100 cheaper, has a bigger library of exclusives, and has a superior online system. And this is coming from someone who only gets to play either consoles because he is too damn poor to afford either one (my lil bro has the 360, my roommate back at my apartment has the PS3).