As a Gm, I find that giving individual 'roleplaying' awards is one of those things that seems like a really good idea at the time but comes back to bite you in the rear down the line unless you balance it out of existance.
Unless the extra xp reward rotates reasonably regularly, what you will get is an xp gap developing between the 'good' roleplayers in the group and everyone else and you'll start getting the situation where they're leveling ahead of other people and that leaves things open for accusations of favouritism and feelings of bitterness. Also, people who never get the 'reward' or consistantly get the smaller rewards would probably not appreciate the weekly reminder that their Gm considers them 'weak' roleplayers.
The situation is improved a little (but not much!) by not making the extra rewards public knowledge but simply informing the recieving player privately. That way, the feeling of missing out by those who don't get it is reduced somewhat and people stay happier for longer.
But I would recommend that, if one feels compelled to give out this type of reward at all, to make it a 'group effort' reward which is shared equally rather than a prize which the players have to compete for. There is, after all, a difference between roleplaying and ham acting.
Yours,
Altin