If you don't want to mess with building one the Dell XPS series are pretty good IMO. I have one that is probably somewhere between 5-7 years old now I think and has been solid for me. I don't do heavy gaming on it as most of mine are older titles and I do more non-gaming stuff with it really. I upgraded it with a SSD hard drive a couple years ago that made a world of difference and probably could just upgrade my 1 TB graphics card to something newer and be fine playing most of the newer games out there. Apex gaming PCs is one that custom builds or has ready to go builds you can buy too and I am sure there are a few more places out there like them too.
Like many said here all depends on what he plans to use down the road. If you buy one already built look at the gaming PC models not the personal or business models as a lot of those don't come with a lot of expansion options anymore especially when it comes to graphics cards because most are integrated graphics and may not even have an expansion port if you buy one of their compact models.