I wouldn't call wanchai computer centre a tourist trap but it certainly may be a little bit more expensive then if you buy it from SSP (Golden center). I wouldn't bother with shopping at Mongkok computer center. It's just not comfortable shopping environment and price is not any cheaper then wanchai.
at SSP (sham shui po) godlen computer center, there are a few shops that are pretty big and you can feel comfortable getting good price and service. you can usually bargain a little bit (but not much) if you pay in cash. the best way to find out what the 'market price' for a computer is to check out several major store.
In terms of stores, i had good experience with Centafield (located at basement of golden centre)
new models, older models, it's all up to you and you usually get what you paid for (and it's fine to pay less if you do not need all the new features) At SSP, you have good selection of old and newer models.
Cheers!
Bruce
ps* If you prefer no hassle big store backing from major electronics chain, you can go to Mong Kok Broadway.
Do your price check @ price.com.hk