The difference with the i7 is negligible, so by the time the i5 becomes too slow, whether that's in 1, 2 or 5 years, depending on your needs, the i7 will have the exact same problem.
Yes, in benchmarks the i7 is faster. But we're talking long renders that'll be a few minutes shorter, nothing that's really noticeable. The i7 does have more video memory, but it's highly doubtful you'll ever use that. And even if you use apps that use that kind of memory, the card itself will become the bottleneck.
It's nice to buy the i7, because 7 > 5, but for most things it's purely psychological. People who "feel" that the i7 is faster overall and with day to day stuff are lucky, but it's all in their heads. Let them compare an i5 with an i7, but tell them the i5 is the i7 and vice versa. You can guess which one will "win".
The difference between the i5 2.4 and i5 2.53 is basically non-existent. So unless you want a bigger hard drive for a premium Apple price, never go for the 2.53-option. Either buy an i7 if you can spare the money and you really want the "fastest" option. If you're not going to use the machine for long renders and real pro stuff and you don't care about the "i7" tag, please save your money and buy the i5 2.4GHz.
Use what you save on e.g. more memory (not via Apple, way too expensive and it's easy to install yourself), Apple Care, or maybe an SSD. Good luck!
