I'd personally just pick up the Corsair 550W unit, it's just around the 450W unit. Both are 80 Plus certified. If you're really against rebates then the 450W unit will handle the HD 4850 just fine.
And yes, the system is going to boot with the crap PSU, no real question about that. It's what will happen with a sudden spike in load that is the question.
And yes, the system is going to boot with the crap PSU, no real question about that. It's what will happen with a sudden spike in load that is the question.