For that 400W PSU, GTX 1050 Ti would be the best GPU you can run with your PC. While GTX 1050 Ti is PCI-E 3.0 x16 GPU, it will run fine in PCI-E 2.0 x16 MoBo since PCI-E 3.0 is backwards compatible with PCI-E 2.0,
pcpp: https://pcpartpicker.com/products/video-card/#c=380&sort=price&page=1