My gaming PC would cost about 2.2-2.3k if I needed to replace everything, guess the base unit alone would be around £1400-1500 but it isn’t particularly high-end tbh, sub £200 CPU, 8gig ram, SSD, decent mobo and soundcard, 400+ quid in graphics cards, I have a 27" monitor and gaming @ 2560x1440 needs quite a lot more GPU grunt over for example 1920x1200 to keep the frame rate over 60 with eye candy turned on.
It was put together with the aim of being reasonably quiet for a air cooled crossfire set up, rather than going for all out performance or value for money, so I spent quite a bit on the case and swapped out the standard case fans for Noctua’s which bumped the price quite a bit.
A good gaming system to run on a single 1920x1080 monitor could be put together for well under £900 and IMO that would be good until the next gen of consoles comes out, as we will not see much in the way of improvements in graphics till then, that’s with quality parts and not skimping at all, on a budget £500- 600 could build a very usable gaming machine.
Higher resolutions than 1920x1200, 3D or Eyefinty see’s the price climb rapidly, if money was no object I would be running something like this: