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Mine fits in a Antec 300 no problems heat really is not a issue certainly not in my case but i guess if you have poor airflow maybe, mine is running on a 550w psu with no problems how ever i am going to upgrade my psu to play safe, but bang for buck....
you are pretty much pushing it there for the PSU might run it now but when the capacitor's start to age something will have to give. the GTX 470 pre factory overclocked I had says you need a 550w minimum PSU.