train from Zhengzhou to Lhasa

I want to travel from Zhengzhou to Lhasa via train, then to Kathmandu. I am a student in Zhengzhou University. Do I need visa to stay at Lhasa for 2-3 days, or minimum 1day? My date of departure will be 15th October 2011. Should I book tickets now? And can I book it online?? And What about baggage allowance?