Given that you're looking to do some more high-end stuff, it might pay off to go the high road and get the 2.66GHz. Here's why:
The 2.4GHz is a great machine, don't get me wrong. In fact, even when you're doing 1080i video editing or organizing high resolution photos, the 2.4GHz is a fine computer. But let's say that 3-4 years down the road, you're still doing the same things (HD editing, etc.) you might be needing a larger hard drive by that point. On top of that, more speed is going to be more helpful particularly as your computer ages.
Now, going the i5 route with a 15" MacBook Pro, you probably would see a difference (faster machine, better graphics card, larger screen obviously). But you're also sacrificing portability. So then it comes down to a question of how useful these extras would be in the long run. I can tell you that from experience, I wish my MacBook Pro was a 13" the 15" is just a little too bulky for my taste.
So it's a trade off. More power for the long haul vs. more portability (and of course, more money). If I had to choose, I'd probably go high end 13" and get the 2.66GHz. Hope that helps.