What PSU do you have right now? What's the brand/model? What are your complete system specs?
Also, I should mention that the X1950XTX is out, and it delivers slightly better performance than the X1900XTX. Connect3D RX1950XTX: $460
They recommend a 450+ watt PSU, and I'd get one with a combined +12V amperage of AT LEAST 30A
. The more amps, the better, and make sure to stick to the good brands.