Additionally, not everyone has the mean or desire to build their own computer. And even if they did, theres no guaranteed way to tell if all the part will work together they way you intend. Therefore, that will always be the advantage of a prebuilt. I've know may people who have built their cars--and are always working on them.
Specialization is the key here folks. Like the folk who started Alienware, if the OP can specialize and cater to the right market, he can make this work.