Hello and welcome to the forums
Here is the comparison list :
T5550 is clocked @ 1.8 with 2mb cache and 667MHZ FSB
T4300 is clocked @ 2.1 with 2mb cache and 667MHZ FSB
T6400 is clocked @ 2.0with 2mb cache and 800 MHZ FSB
T6500 is clocked @ 2.1 with 2mb cache and 800MHZ FSB
So T5600 is the best option here,the difference between the T4300,T6400 and T6500 isn't much but all of them are 300MHZ faster than a T5550 which again doesn't make a lot of difference in most apps,but in CPU limited apps/games u will benefit from the extra 300MHZ