That depends on the age of the machine (a pentium 4 is also very demanding) but for a single 58xx card usually a 400-500 watt psu has a sufficently sized 12v rail, one like this is on the lower end of the spectrum it would handle a single 58xx or a pair of 57xx without too much trouble http://www.newegg.com/Product/Product.aspx?Item=N82E16817182202