It says on the Nvidia site that the min wattage needed for the ultra is 500W. That being said it all depends on your PSU and whether it is a reliable and quality brand and also, how many amps there are on your +12V rail(s). I think that the ultra needs around 32A but I'm sure someone else can correct me if I'm wrong.
Get a Enermax 850w Galaxy if your budget will run to that. I have a BFG 8800ultra running with this PSU and its great. However silverstone's psu are not that bad, so you may be ok running that.
According to this http://www.tigerdirect.com/applications/searchtools/item-detailsInactive.asp?Sku=S35-1150 , if this is your power supply, then you have 54 amps on the 12 volt rails, which is plenty for any card made. Amps equals watts divided byvoltage or at 12 volts with 600 watts you have 50 amps. If the voltage changes, you divide that into the wattage to find your amperage. Say, 3 volts with 600 watts = 200 amps etc
depends on what processor you have, but for most cases it would be enough , I'm running an overclocked gtx , a q6600 @ 3.8 and 4 hard disks with a 650psu .. so i'm guessing you won't have a problem