People are using Premiere on much worse hardware. If you think the $250 is high, then it probably isn't a great choice for you.
250 is high for 300mhz I'd say. But don't really know the real life differences. Actually I'm used to editing on my Air (1.8 i5, 8gb RAM), so I'm guessing any high-end Mac will make a difference.
So the general consensus seems to just get the base model and forget about the 300mhz?