I've been doing this since the 1980s.
IMHO it depends on whether you are doing it for a friend and will thus end up supporting the product
If yes, then I recommend a Dell. They work OK and performance wise as as good as anything of the same CPU spec and RAM size. Like all high street vendors they use fairly cheap mechanical components but you get a warranty and "some sort" of telephone line & service, even if staffed by apes (they all are these days).
If no i.e. you want it for yourself then I would buy the parts. A decent expensive motherboard (they are so cheap that going for a good one is OK), a decent large diameter low speed (silent) CPU cooler, a big 500W power supply with temperature controlled fans, a high end video card, and a fast SATA hard drive (not necessarily big; usually the only way to fill say 200GB is with p0rno movies). A top end Plextor DVD writer. And I fit a high quality £30 ball bearing fan into the case, replacing the £1 plastic one. The cheap CPU fans invariably seize within a couple of years but a good quality CPU cooler should be OK. And a high quality case to put it all in - these come from specialist computer parts shops; the high street cases are all absolute cr*p. This way it will cost a lot more than a Dell but you get better parts.
Once it is built, and the basic software installed and working, I put Trueimage on it and make a recovery CD and an image snapshot (which goes on a DVD). I do this for PCs I do for others too, and that way when (not if) they trash it, you can easily get them back to what they started with.