The best GPU

the GTX 1080ti is the best consumer GPU right now. You could try and go for a 1060 6gb which is around 260-300 USD, or go for a used GTX 980ti for $300~ which is the equivalent of a GTX 1070 just consumes a bit more power and isnt as efficient. New GPUs are coming out next month(probably 1170 and 1180) and are supposed to be really really good so you could save up a bit more and try and wait for those. Higher end GPUs are still a bit inflated due to memory shortages and mining but if you have the budget for them go ahead.