Hey Daave,
I spent 6 Months in China from Aug till start of Feb. The weather turned pretty horrible around the mid of Dec and just got worse from there. (I was there when they had one of the worst winters in over 10years however...)
It gets really cold from central China all the way North. However Yunnan, down south, was pretty good. South of Kunming (capital of Yunnan) is on the boarder with Laos and the weather there is ok that time of year. Even in Kunming it was sunny most days.
I guess the best advice is to work out were you want to go and then work out the altitude. It plays a massive part in whats cold or climbable that time of year. Higher Alitude = colder. And when I say colder I mean at around 4000m it was normally in the negatives all the time...
Either way you'll have an Awesome time!
S