not much. both have the same core, 2MB L2 cache and 667Mhz FSB. the only difference is the clock speed. T5450: 1.66Ghz T5550: 1.83Ghz
About 200
Should easily be able to. I'm playing it on my PC with inferior specs, and I can run it on premium (At 30~35 fps). Though, it does also depend on your OS. I was running XP, which doesn't use nearly as much system resources as Vista. I'm not sure if you can use the premium graphics with Vista.