Cheap cheap 300-400 pc. Lighter games.

Hello, jj_foxx!

Well, it's possible to get a low-end gaming PC with monitor for 400 pounds, but, having OS included will hamper you performance a lot. I would strongly suggest you to download Windows 10 Technical Preview and use it for free, or download Windows Trial and save up in the 90 days.

I personally wouldn't go lower than this:

PCPartPicker part list / Price breakdown by merchant

Type Item Price
CPU Intel Pentium G3258 3.2GHz Dual-Core Processor £50.34 @ Aria PC
Motherboard MSI H81M-P33 Micro ATX LGA1150 Motherboard £33.37 @ Scan.co.uk
Memory Crucial 4GB (1 x 4GB) DDR3-1600 Memory £21.24 @ Amazon UK
Storage *Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive £38.34 @ Aria PC
Video Card XFX Radeon R9 270 2GB Double Dissipation Video Card £110.72 @ Scan.co.uk
Case Zalman Z3 Plus ATX Mid Tower Case £26.99 @ Amazon UK
Power Supply XFX ProSeries 450W 80+ Bronze Certified ATX Power Supply £38.94 @ Aria PC
Monitor BenQ GW2255 60Hz 21.5" Monitor £80.40 @ Kustom PCs
Keyboard Gigabyte GK-KM6150 Wired Standard Keyboard w/Optical Mouse £12.12 @ Dabs
Prices include shipping, taxes, rebates, and discounts
Total £412.46
*Lowest price parts chosen from parametric criteria
Generated by PCPartPicker 2015-04-12 00:19 BST+0100

OS left out, you will have a pretty good entry-level gaming, even capable of some AAA's in medium 60fps/high 30 fps.

Hope it helps! =)
Feel free to ask anything.

/r/buildapcforme Thread