well, i actually asked that same question about a year ago but my situation was a little different. I was able to crossfire a 4870 and a 4850 using that power supply. It worked but i only ran it for about 3 weeks as i was waiting for my new 610-watt PSU so i might have been lucky. I would imagine that even if it worked you would not want to make it a prolonged setup and definitely get a decent PSU right away
Your supposed to need 30A for a complete system and one 4850. Adding another should put you close to 40A. I don't think the 450W has that much. Might be able to run it for a bit, but I doubt you'd see long term success.