So yeah , can we trust them ? I'm building a new system and when i max everything it says i need 400 watts , so am I safe to buy a cooler master GX450 ?
To a degree you can trust them, but it would be good to double check that the GPU+CPU doesn't consume more than your PSU's 12v rail will supply. You have a 50watt buffer zone over the requirement, which should be fine.
If your concerned though, you should list your specs. Worth noting that I have heard bad things about cooler master psu's, but I haven't ever used one so I'm just parroting somebody; Seasonic are nice PSU's as and I've heard good things about Corsair.
It doesn't hurt to have a few extra watts if price isn't a major concern, but don't sacrifice quality for extra power if you are cost constrained.