Just shop around for the best quality. A lot of places do complete machines, but they all vary, skimping on some parts as regards quality, opting to put cheaper components in to keep costs low.
OCUK has had varying reports of the way they build machines, some people saying they are rubbish others saying they are OK.
Any machine whether custom built by a supplier or by yourself should have the same criteria, quality.
Look for either good machines off the shelf, not specifically gaming machines either, as the name tag will want some to increase the price tag, or machines that are given multiple options of your choosing.
What sort of price range are you talking about ?, as this has a big impact on the type of machine.
Couple of places I have used in the past, both having some good deals at times.http://www.eclipsecomputers.com/landing ... ge=systems
Look at their prebuilt and self build sections.http://www.dabs.com/
I used these for purchasing my gfx card as I got a good deal, but like anything component wise, searching around is a good option as long as you take delivery costs into account, when purchasing multiple items.