This is what i could find out about the CPUs you mentioned:
Pentium T4200, SU2700
T4200 DC 45nm 2.00 Ghz FSB800 1MB
Pentium Dual Core Mobile,
T3400 DC 65nm 2.16 Ghz FSB667 1MB
T4300 DC 45nm 2.10 Ghz 800 1MB
Core 2 Duo T6500, T6400, T6600, T5800
T6500 DC 45nm 2.16 Ghz FSB800 2M
T6600 DC 45nm 2.20 Ghz FSB800 2M
T6400 DC 45nm 2.00 Ghz FSB800 2M
T5800 DC 65nm 2.00 Ghz FSB800 2M
Celeron Dual Core
T1600 DC 65nm 1.66 Ghz 667 1M
N280 1.66 Ghz FSB 667 512
Z520 1.33 Ghz FSB 533 512
Now it would all depend on what you want and the price of them....
To be fair i still have a P3 laptop which i use for internet and some other things...it's struggling but more because of the lack of memory. So it's not like you have to have the best copmputer in the world. I would look more at price, aesthetics and what else you would get with it, webcam, type of screen et et etc.
Theoretically out of the CPUs you mentioned the T6600 is the best.
Hope this helps
I have some issues with Dell regarding T5800 vs T4300. I ordered a Studio hybrid with T5800 around 7 months ago and it could NOT play buy-ray disk well. I spent 6 months with them debugging softwares, and replacing parts. Finally they sent me a replacement computer couple days ago. However, it still does NOT play blu-ray daisk. Tthe biggest thing disappointing me is that the processor in the replacement computer is now T4300. The tech support refered this link to me and told me T4300 is faster than the original T5800.I am furious. I just don't like being cheated like this. Isn't Intel T5800 newer and superior than T4300 regardless the tiny speed difference?