It depends what you are using this PC for. If it is for gaming or very heavy applications, I would suggest an i5. You can always look at the lower priced i5's. The i5-3330 is typically $25 less than the i5-3570. While that really isn't alot of savings, you can also save some money in your motherboard selection. If you do not intend to overclock you could get an H77 motherboard rather than Z77 to save some money in combination with a lower clocked i5.
Secondly, I would also look at the AMD FX-6300 which is 6 cores and performs between the i3 and i5 depending on the application of game. You can also get the AMD motherboard relatively cheap.
I would consider the above 2 options before lowering to an i3 unless you really need to keep the price down. The i5 will run pretty much whatever you throw at it as it as 4 cores over the 2 Hyperthreaded cores in an i3.