The rear view indicates the included PSU has a little voltage switch. At best, it is an obsolete, inefficient design. At worst, it is overrated, possibly by enough that it will not be able to power your card. Based on my arbitrary assumptions about # of fans and other devices, the PSU calculator at http://www.extreme.outervision.com/PSUEngine
suggests 331W. Even if mildly overrated, your PSU should be able to handle it, although it may be under stress.
The 500W Antec Earthwatts is available at a good price right now, $40 after MIR: http://www.newegg.com/Product/Product.aspx?Item=N82E16817371035
and the 430W version has a similar deal: http://www.newegg.com/Product/Product.aspx?Item=N82E16817371034