For the video card you selected the minimum PSU recommended is 450 Watts. Manufacturers recommend this under ideal conditions; almost lab conditions.
To provide for aging of the caps, some future changes/mods, etc., I would go for a 600 to 650 Watts PSU. Watch Newegg's Shell Shocker deals; now 3 deals daily, for some great prices for PSUs in this range. Do buy a good brand like Corsair.