OK, and a little rough advice from me...

If you still have this problem, I`d suggest sth like that: try importing the whole scene into a new file and then render it. Maybe you have some odd setings in your file. Your render times are enormous for a <1M vert project-my current proj takes some 8 mins to render on internal, 800x600 with 5x oversampling, though it is some 2,5 mln vert (Celeron 1,4, 0,5GB mem). Importing everytghing into a new one is my "Last line of defense" when everything goes wrong:):)