Best gpu for 300-400$

Solution
390X and here's a couple in that range.

PCPartPicker part list / Price breakdown by merchant

Video Card: Asus Radeon R9 390X 8GB Video Card ($389.99 @ Micro Center)
Total: $389.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-01-06 17:17 EST-0500

or

PCPartPicker part list / Price breakdown by merchant

Video Card: Sapphire Radeon R9 390X 8GB Tri-X OC Video Card ($373.98 @ Newegg)
Total: $373.98
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-01-06 17:18 EST-0500
You can find used GTX 980's on eBay, right now, in this range.... including Asus' Strix 980, EVGA's SC w/ ACX 2.0 980, & MSI's GAMING 4G GTX 980s. I even see a Gigabyte G1 Windforce 980 - that sold for $386.00 on 12/31/2015
 

Dark Lord of Tech

Retired Moderator
390X and here's a couple in that range.

PCPartPicker part list / Price breakdown by merchant

Video Card: Asus Radeon R9 390X 8GB Video Card ($389.99 @ Micro Center)
Total: $389.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-01-06 17:17 EST-0500

or

PCPartPicker part list / Price breakdown by merchant

Video Card: Sapphire Radeon R9 390X 8GB Tri-X OC Video Card ($373.98 @ Newegg)
Total: $373.98
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-01-06 17:18 EST-0500
 
Solution
Definitely the R9 390X. I'm an Nvidia owner and would recommend it over even a 980 which is more expensive yet they trade blows with each other in various games (even though the 980 has 4GB of VRAM to the 390x's 8GB), so there you go. The 390x does use a lot more power than a 980 though, and the 980 is an excellent overclocker which can push it above a 390x, so you have to factor that in as well.