The difference is that T4300 is clocked @ 2.1 with 1MB cache and 800MHZ FSB whereas T6500 is clocked @ 2.1 with 2MB cache and 800MHZ,the difference between these 2 isn't noticeable in almost every game/app with the exception of some few apps which benefit from a higher cache.
If other specs are the same go for the one which is cheaper.