............Unless you really know what you are doing,
But that's pretty much the point doing a self build. If you don't when you start you pretty soon will because it's easier than you expect and empowering.
There is a one off cost in terms of a little bit of research and you don't save much money the first time but what you gain in confidence and a sense of control is immense. PC breakdown fears become a thing of the past. No individual part of a PC is particularly expensive and it become s doddle to spot which one is fucked and replace it. Job done.
It's easy to find a magazine, website, forum, friends to help with specing up and even if you are very unfortunate provided you can demonstrate you have made some effort yourself you will find peer help and support on forums far better and convenient than any manuafcturers 'helpdesk'.
How much do you want to spend? What do you want to do with it? How long do you expect it to last? Any special requirements?
^^