I always used to build or uograde myself, but now days the big players can buy the components with such a discount its just not worth going down the DIY route anymore.
Looking in magazines like MicroMart which has hundreds of adds each week from part suppliers, adding up the cost its maybe the odd £50 cheaper than buying from, say, Mesh, Evsesham, Time etc ...
At least buying a complete system you are avoiding the almost enviteable conflict of some parts that always seemed to happen building myslef. Plus companies like Mesh tell you excatly what the put in their machines, make and model of motherboard etc, and you can read the reviews on sites like
www.tomeshardware.com to decided what is the better machine based on its spec ....