An i5-2400 is right out, it's a different socket altogether and you'd need a new motherboard and if you were going to buy a new motherboard, you shouldn't go with such an old platform.
One problem is that it's an old OEM motherboard with poor documentation from a defunct brand bought by Acer. Given that this model sold with an i5, it's likely that the 73W Westmere i5 CPUs (650, 660, 670, 680) will work. But there's no guarantee as these types of companies don't always let their BIOSs read upgraded CPUs; it's much more profitable for them for you to buy a new PC from them rather than upgrading an existing one.