This should be enough, tried to keep the cost as low as possible without sacrificing component quality.
The 6300 should steam quite decently at 720p one overclocked, and the r9 280 will be more than enough to go high settings at this resolution. Also switched the network adapter to pci since all pci-e x1 slots will be occupied with the video card installed, but you'll probably want to use a USB 3.0 adapter or a wired CAT 6 connection if you can, since connection quality degrades quite a bit once you go wireless, and that can be detrimental to your stream.
PCPartPicker part list:
http://pcpartpicker.com/p/XPQRbv
Price breakdown by merchant: http://pcpartpicker.com/p/XPQRbv/by_merchant/
CPU: AMD FX-6300 3.5GHz 6-Core Processor ($99.99 @ Newegg)
CPU Cooler: Cooler Master Hyper T4 70.0 CFM Rifle Bearing CPU Cooler ($19.99 @ Micro Center)
Motherboard: Gigabyte GA-78LMT-USB3 Micro ATX AM3+ Motherboard ($46.98 @ Newegg)
Memory: G.Skill Ares Series 8GB (2 x 4GB) DDR3-1600 Memory ($53.10 @ Newegg)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($46.99 @ NCIX US)
Video Card: XFX Radeon R9 280 3GB Black Edition Double Dissipation Video Card ($159.99 @ Newegg)
Case: Cooler Master N200 MicroATX Mid Tower Case ($39.99 @ NCIX US)
Power Supply: XFX TS 550W 80+ Bronze Certified ATX Power Supply ($26.99 @ Newegg)
Wireless Network Adapter: TP-Link TL-WN851ND 802.11b/g/n PCI Wi-Fi Adapter ($19.81 @ Amazon)
Keyboard: Cooler Master CM Storm Devastator Gaming Bundle Wired Gaming Keyboard w/Optical Mouse ($26.99 @ NCIX US)
Total: $532.82
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-04-16 01:16 EDT-0400