What should I upgrade in my computer?

Solution
In that case, I'd go for something like this if you're happy to go team red on the graphics card:

PCPartPicker part list / Price breakdown by merchant

CPU: AMD FX-8350 4.0GHz 8-Core Processor ($158.99 @ SuperBiiz)
CPU Cooler: Cooler Master Hyper 212 Plus 76.8 CFM Sleeve Bearing CPU Cooler ($25.98 @ OutletPC)
Motherboard: Gigabyte GA-970A-DS3P ATX AM3+ Motherboard ($60.98 @ Newegg)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory ($52.99 @ Newegg)
Storage: Corsair Force LS 120GB 2.5" Solid State Drive ($74.99 @ Amazon)
Video Card: XFX Radeon R9 290X 8GB Double Dissipation Video Card ($394.98 @ Newegg)...
You have only 120GB of storage. Are you recycling a HDD from elsewhere?

Also, what do you want it for? As a gaming build for 1080p it's fine. As a 4k build it might not be. As a video editing/animation PC it could do with (maybe) more RAM and processing power. It all depends on the intended function.
 
In that case, I'd go for something like this if you're happy to go team red on the graphics card:

PCPartPicker part list / Price breakdown by merchant

CPU: AMD FX-8350 4.0GHz 8-Core Processor ($158.99 @ SuperBiiz)
CPU Cooler: Cooler Master Hyper 212 Plus 76.8 CFM Sleeve Bearing CPU Cooler ($25.98 @ OutletPC)
Motherboard: Gigabyte GA-970A-DS3P ATX AM3+ Motherboard ($60.98 @ Newegg)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory ($52.99 @ Newegg)
Storage: Corsair Force LS 120GB 2.5" Solid State Drive ($74.99 @ Amazon)
Video Card: XFX Radeon R9 290X 8GB Double Dissipation Video Card ($394.98 @ Newegg)
Case: Rosewill STEALTH ATX Mid Tower Case ($79.99 @ Newegg)
Power Supply: Antec Neo Eco 620W 80+ Bronze Certified ATX Power Supply ($64.99 @ SuperBiiz)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($14.98 @ OutletPC)
Operating System: Microsoft Windows 8 (OEM) (64-bit) ($92.95 @ OutletPC)
Wireless Network Adapter: Rosewill RNX-N150PCe 802.11b/g/n PCI-Express x1 Wi-Fi Adapter ($11.59 @ OutletPC)
Keyboard: Cooler Master Storm Devastator Keyboard & Mouse Wired Standard Keyboard w/Laser Mouse
Total: $1033.41
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-05-03 07:57 EDT-0400

Note: I can't be sure of whether the card will fit, the card is 11.61" (295mm) and the case is 19.65" x 8.23" x 19.65"
 
Solution