Hi there,
The other guys are otherwise engaged at the moment so I hope I can be of help.
Ati recommend at least a 450w power supply for the Radeon X1950 Pro (see link) with 30A output on the 12v Rail.
And yes, insufficient power can affect file transfers; indeed it will affect every operation of the computer, and can damage your components.
Here's a link to the spec of your video card with it's recommended power requirements etc.
http://shop.ati.com/product.asp?sku=3122113
Here also is a link to a Power Supply Calculator.
http://www.extreme.outervision.com/PSUEngine
Please add 30% to your total, as recommended at the foot of the webpage.