1 .How much money are you willing to spend ?
2. What are you going to use it for ?
3. Do you need a fast machine for gaming or video editing ?
5. If you don't need a fast machine but get at least a dual core machine ( 2 separate CPU's).
4. Do you want portability get a laptop ?
5. Where do you live ? Best to get it near you house ?
In Hong Kong 298 Computer Mall in Wanchai. In TST Sham shui po-Golden Arcade.
If you are cheap like me I get a good use one. In three years, your computer will be worth half as much. Tried craigslist.org for used one.
http://hongkong.craigslist.hk/sys/