T6400 – 2 GHz 2MB L2 cache , 800MHz FSB, 45nm (lower better), 35watts, Penryn Family of processors.
T5800 – 2 GHz 2MB L2 Cache , 800MHz FSB, 65nm(lower better) , 35watts, Merom Family of processors
And both the models support native 64 bit computing
T5800 was the popular model of Intel's midrange laptops but it has got a upgrade with a new T6400 processor after the 2008 Christmas season. Almost all the major laptop brands have started sidelining their T5800 models in favour of T6400 with almost the same spec.
Many have asked me what is the difference between T5800 and T6400. Typically should be no major difference between the two, but if you have the choice and the price difference is less than or around 50$ always go for the newer model(T6400), the newer models have better architecture better support and your laptop might get a better resale value if you decide to sell.
The only difference in terms of specs is T6400 follows 45nm technology and belongs to the penryn family, whereas the T5800 follows the 65nm architecture.
cpu benchmark (cpubenchmark.com) values of a very small sample of T6400 show a markedly high improvement over the T5800, im not sure if this is because of the overall higher configuration associated with current T6400 based laptops or maybe inaccurate performance measurement at cpu benchmark. I haven't included the benchmark values as it could be misleading.