International price comparison site
It also depends on where you live, stuff seems to be cheaper in the States. Also make sure that everything is compatible with eachother.
What you should keep in mind is:
What do you want to use the computer for
If you walk in a store and they don't ask you this question in the first 5 minutes, walk out the store, for they are probably only thinking of sales instead of tailoring the PC to your needs... (Used to be a PC salesman for over 4 years and in 9 out of 10 cases this is the first question I ask so I can think of what the customer needs, rather than just selling as much as possible)