I have wondered that too, why PSUs have been moved to the bottom. It seems that PSUs had extra fans, so those were utilized to help move heat out of the case, even though it wasn't most efficient for cooling the PSU itself. With higher wattage power supplies, the PSU needs fresh, cool air, not air already heated by the other computer components.
That being said, my ATX case with the PSU on top is fine with a 630 watt supply.
I have been impressed with the Antec -hundred line of cases though, particularly the Antec Nine-hundred II case. That might be a bit higher than your price range though.