Oh, I see. Well, most of the advice still applies, as most PC/electronics shops offer ready built gaming PCs for various purposes and budgets. It does pay to know what exactly you are looking for. So when you look at a selection look at the ones that have a better GPU in the price range that you are considering, etc. Some online shops have configurators where you can adjust your gaming PC to your needs and budget.
What country are you in? Or just goolge 'gaming pc online shop <your country>' and compare the results for price and quality of the components. In my country a ready built PC is about 500 USD or 400 EUR more expensive than the individual components separately. For my PC that would be about 1/3rd of the price. On the upside getting a ready-built PC you are guaranteed to get a nicely adjusted configuration and you don't have to wait for the individual shipments to arrive and it's probably easier in case something doesn't work too.
I'd go for a quality online shop with a good reputation in your country, not one that opened a few months ago. I wouldn't buy a gaming PC in a huge chain like Mediamarkt or Radioshack, but that's my personal preference.
Heres an example for a
configurator in the UK