What psu do you have?
Probably a amd R9-390 can be found, but they should have 650w.
On the nvidia side, a GTX970 will exceed your budget at around $300, but one of those will only need 500w.
A strong GTX960 will be under $200.
Seems to me that nvidia has a price gap between $200 and $300 that needs to be filled.