I would say a PSU with max 400W is enough. You can go higher, but I would suggest 300W as an absolute minimum (otherwise it won't protect you if power goes out during gaming). Be warned: the model number is usually with VAs. Watch for W.
I would estimate you system draws between 80-250W (my i5 with 5850 tops 270W), with 30-40W additionally for the monitor.