You could always by a rebuilt PC but there is little to no doubt that he's not going to get the most bang for his buck. So if he has the extra money he doesn't care about and wants to blow it on a overpriced computer, then that's an option. Building a PC is his other option. However it's not for everyone. Building a PC isn't a hard task at all at it's core, but there is a lot of research/understanding that comes along with it. It's all a matter if he want's to put in the time and energy.

Here's some links to two site's nearly everyone uses when it comes to a PC build.
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You can try pc specialist. You can build any pc or buy pre build one.

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Imo it's better if you are able to build it your own, parts are cheaper and you'll have the feeling of accomplishment once it's completed and booted up.

There are a lot of Price-guided builds online as well if you want to look those up on PCPartPicker etc.

Thanks everyone

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Significantly better than any pre-built you'd get for that price. In fact, this £800 rig would be comparable to something like a £1300+ pre-built. Building a PC is not hard, at all, and if you buy a pre-built, you'll get low end, low quality components for your budget and a lot of the time, a terrible case and PSU.

Please sway him off buying a pre-built. It would be better for his system and his wallet.

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I found this site after I built mine. But it looks incredibly simple to use.

i got my gaming laptop from xoticpc, but before you buy just browse the website and the reviews of the website and such. My friend had a bad experience from ibuypower but he never did his research. Anything with an i7 4th gen or better will be great, and a gtx 770 or higher will run a lot of stuff on high and the newest games on medium