Newegg.com has very good prices. Tigerdirect.com is another name that comes to mind, but I've never purchased anything from them. You might be able to get really good deals at physical stores if they happen to be offering rebates on the components you're looking for. I don't know about places in Hong Kong, but over here in the States you can probably find good rebate deals at places like Circuit City, CompUSA, and BestBuy.

By the way, unless you have a specific reason to buy an Intel Core 2 Duo (I assume you don't mean the original Core Duo), you might want to check out the AMD Athlon 64 X2. It's a series of dual core CPUs that probably outperform yet undersell their Intel counterparts. A good place to start searching for the best deal on them might be here.