If your going to Hong Kong that will automatically renew your visa for another 3 months that is of course if you have 3 month multiple entry visa. If your Visa is for a maximum of 3 months then you will need another one.

I want to say "no", you won't need a new visa but when I read your post here I couldn't tell precisely when you are returning to China.  Your current visa, which I would venture to say might be a "L" or an "F" visa expires in March -- if you return to China before the visa expires, you will be re-admited provided however that your visa is a multiple entry visa for China.  If your visa is a single entry visa for China, then yes, you will need a new visa.  If your visa is a multiple entry visa, then no, you won't need a new visa.  I would need some more details to make a more accurate assessment. 

It's singel entry and I'll only stay in Taiwan for a few days. So I guess I'll need to go to the visa office in HK

Indeed, you will -- since it's  a single entry, your exit to Hong Kong and Taiwan cancels the visa.  Just remember, however, if you are going next week (week of Spring Festival, 2012) that the Hong Kong Visa Office will be closed for a good part of the week, so perhaps you might be better off to pick it up on your return.

Looks like they are open on thursday & friday (on their website they have the closingdays of 2010!). But will it be a nightmare to do this those days??