To be honest, I think you should save up just a little more money. You will be a lot happier with the final products and be able to run games significantly better.
Something along these lines: PCPartPicker part list / Price breakdown by merchant
| Type | Item | Price |
|---|---|---|
| CPU | Intel Pentium G3258 3.2GHz Dual-Core Processor | $49.99 @ Micro Center |
| Motherboard | ASRock H81M-HDS Micro ATX LGA1150 Motherboard | $49.99 @ Micro Center |
| Memory | PNY Optima 4GB (1 x 4GB) DDR3-1333 Memory | $42.99 @ Micro Center |
| Storage | Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive | $49.99 @ Micro Center |
| Video Card | XFX Radeon R9 280 3GB Double Dissipation Video Card | $169.99 @ Micro Center |
| Case | Thermaltake VL80001W2Z ATX Mid Tower Case | $19.99 @ Micro Center |
| Power Supply | EVGA 500W 80+ Bronze Certified ATX Power Supply | $29.99 @ Micro Center |
| Prices include shipping, taxes, rebates, and discounts | ||
| Total (before mail-in rebates) | $467.93 | |
| Mail-in rebates | -$55.00 | |
| Total | $412.93 | |
| Generated by PCPartPicker 2015-02-15 13:12 EST-0500 |
This will offer you significantly better performance than the above build. You would pay a little bit more and sacrifice 8gb of ram but get a much better graphics card in the process. You would be able to add another 4gb of ram in the future when you had more money to spare.
As for operating system, you could try to get a copy of windows from /r/microsoftsoftwareswap or use a linux distribution for the time being.
For a cheap keyboard and mouse the CM Storm Devastator kit will get the job done for the time being and is about as good as you're gonna get for the price: http://www.amazon.com/CM-Storm-Devastator-Keyboard-Edition/dp/B00DKXXAAQ/ref=zg_bs_318813011_5
For a monitor, if you don't already have one laying around then hooking it up to an hdtv would be a perfect solution for the time being.