The biggest factor that will answer your question is your budget. If you can comfortably afford to go for the i7-860 then go for it, if not then the i5-750 would be a good choice. Both perform extremely well in gaming and should handle anything you throw at them.
Another thing to consider is the socket type. The i5-750 and the i7-860 both use the LGA1156 socket whereas some of the 'higher end' i7 chips use the LGA 1366 socket. Would it be worth you using a board and processor with the 1366 socket type in case you ever want to upgrade to the hexacore chips in the future?