So besides the 750 model being quad core, there is completely different architecture involved. i.e. the way i5 and the motherboard and ram are linked on socket LGA1156 compared to the socket 775 motherboard and chips.
Well for one, the T6600 is a mobile unit and i5s are desktop unless you are asking about the mobile variants that might hit.
The T6600 is based on the older Core2 arch while the i5 is Nehalem based, which is newer.