Can anyone tell me what is the best way to travel from Beijing to Shanghai?

Generally speaking, the train ticket fare is much cheaper. For the airplane ticket fare, it will not be cheap during this week.
I suggest to take bullet trains from Beijing to SH, which will cost you about almost 9 hours. The second class seat ticket fare is around 408RMB and 650 RMB for the first class seat.
If you will stay in China for a long time, it is suggested to buy a new one there. If you just traveling there, you had better take your old one. As far as I know, people still cannot get the VAT refund in SH currently.

As a foreigner living in China, based on my experience is better to take the bullet train from Beijing to SH. Last year took me 5.5 hours and 695 rmb for the 2nd class seat and 950 rmb for the first class. Everyday the cost will be the same.
I think the cost will be pretty much the same this year (more or less) and the normal rate for plane ticket would be around 1200 - 2000 rmb, and the journey will take 2 hours. Considering you have to wait for another hour before departure and the journey to the airport, bullet train is only one hour longer and cheaper. It's convenient too.

For the second question, depends on what brand do you want to buy. If it's international brand like Apple, Asus, Fujitsu, etc, the price is much higher. VAT only available for those who will leave the country as soon as they buy the computer, not for those who will stay in China. But have to say that they do have some promotions which make the price lower than other countries, but not much. HK is the place if you want to buy the electronics stuffs.

thanks very much for answering my questions,Its fantastic and its exactly what I required.Bye for now.