For $400 I recommend the following to make sure you have no issues when upgrading to your new graphics card.
PCPartPicker part list:
https://pcpartpicker.com/list/7hn8f8
Price breakdown by merchant: https://pcpartpicker.com/list/7hn8f8/by_merchant/
CPU: AMD - Ryzen 5 1400 3.2GHz Quad-Core Processor ($154.99 @ SuperBiiz)
Motherboard: ASRock - AB350M Micro ATX AM4 Motherboard ($40.98 @ Newegg)
Memory: G.Skill - Ripjaws V Series 8GB (1 x 8GB) DDR4-2666 Memory ($81.99 @ Newegg)
Case: Corsair - Carbide SPEC-04 (Black/Red) ATX Mid Tower Case ($49.99 @ Amazon)
Power Supply: SeaSonic - EVO Edition 620W 80+ Bronze Certified Fully-Modular ATX Power Supply ($65.98 @ Newegg)
Total: $393.93
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-10-20 20:06 EDT-0400
Upgrade to 16GB of RAM if you can afford to later.