The are a lot of pros and cons with self build and pre built.
The great thing about Dell is if you machine goes tits up you will have a engineer on site next day with whatever parts are required, so if it was for a business id defo say go for Dell.
If you build your own (especially gettin different parts from different stores) and somthing goes pare shaped you run the risk of one company blaming another company etc etc.
A good place ive found is
www.ctidirect.co.uk they are cheap for components and you can choose the exact components you want in your pc and for £30 they'll put it all together for you, and from personal experiance they are good when you have to return things!!
EDIT - ooops for £35 they will put it together for you!!!