Solid state disk to boot from - say 250Gb, can have a bigger rotating disk to hold other stuff on if required later (just make sure there is room to fit it)
Personally I like Nvidia Graphics cards - but there are sites that publish benchmarks (mine is a Gigabyte GTX660 - gets five stars in reviews)
It probably needs to run windows for maximum compatibility with most games - I prefer Windows 7 but 8.1 is usable. Probably should use 64 bit version.
16Gb fast memory. Processor - Hmm. A faster dual core used to be better than a slower 4 core.
I use a 32inch full HD TV (1920 x 1080) as a computer screen, would probably be wicked for games playing.
All that said, he'll probably want a laptop to take to friends houses.
Everything I've said could well be carp.
(alianware is Dell owned isn't it?)