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.