You will need more than $300 to build a decent gamer. Win 7 alone is anywhere from $85 to %100. I presume you already have a monitor, KB, mouse, and some kind of speakers or headphones. Here is an example I came up with for the bare minimum to be able to game at 1080p resolution at medium settings.
[PCPartPicker part list](http://pcpartpicker.com/p/FyznjX) / [Price breakdown by merchant](http://pcpartpicker.com/p/FyznjX/by_merchant/)
Type|Item|Price
:----|:----|:----
**CPU** | [Intel Pentium G3258 3.2GHz Dual-Core Processor](http://pcpartpicker.com/part/intel-cpu-bx80646g3258) | $69.99 @ Newegg
**Motherboard** | [MSI H81M-P33 Micro ATX LGA1150 Motherboard](http://pcpartpicker.com/part/msi-motherboard-h81mp33) | $27.99 @ Newegg
**Memory** | [Team Vulcan 8GB (2 x 4GB) DDR3-1600 Memory](http://pcpartpicker.com/part/team-memory-tlyd38g1600hc9dc01) | $56.99 @ Newegg
**Storage** | [Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive](http://pcpartpicker.com/part/seagate-internal-hard-drive-st1000dm003) | $48.49 @ OutletPC
**Video Card** | [Asus GeForce GTX 750 Ti 2GB Video Card](http://pcpartpicker.com/part/asus-video-card-gtx750tioc2gd5) | $116.98 @ SuperBiiz
**Case** | [Rosewill REDBONE ATX Mid Tower Case](http://pcpartpicker.com/part/rosewill-case-redbone) | $24.99 @ Newegg
**Power Supply** | [EVGA 430W 80+ Certified ATX Power Supply](http://pcpartpicker.com/part/evga-power-supply-100w10430kr) | $15.99 @ Newegg
**Optical Drive** | [LG GH24NSC0B DVD/CD Writer](http://pcpartpicker.com/part/lg-optical-drive-gh24nsc0b) | $12.99 @ Newegg
**Operating System** | [Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit)](http://pcpartpicker.com/part/microsoft-os-gfc02050) | $84.15 @ SuperBiiz
| | **Total**
| Prices include shipping, taxes, and discounts when available | $445.56
| Generated by PCPartPicker 2014-11-25 17:53 EST-0500 |