If you are looking at doing this for a christmas present, then i would say do it BEFORE christmas.
REASON :- Most of the computer industry closes down from chirstmas to at least the new year. After the new year, most computer distribution channels are running on the skeliton crew. So you expect delays on getting stock in. Even shortages too as the manufactures.
And just when every thing is back to normal production pace in the IT Indrustry.... BANG! Chinese New Year when Asia just shuts down for up to 2 weeks.
And by the time that Chinese New Year is over, a lot of the manufactur's have released new models of motherboard or video cards.
I have been working in the computer industry for the last 10 years now, and i have always been this way for me.
However, even with purchasing the computer before new year, make sure you have a look at it and use it before chirstmas. There is nothing worse that buying a new computer for chirstmas, opening it and finding out there is somethign DOA. Because getting ANY REPLACEMENT PART during the pariod between Christmas and 2 weeks after new years is pain as a lot of people have taken holidays.
In a nut shell, either buy your computer before chirstmas, but make sure that you use look at it and use it at least once before Dec 24. Otherwise wait until after Chinese New Year at take advanatage of the newer Technology that gets released. (or the price reductions on the older technogy.
The only valid reason I see for purchasign a computer right after chirstmas is to take advantage of the after christmas sales that some of the retail chains are promoting.