Obviously the biggest differences between them are in Blu-ray, graphics and company. I don't think there'll be a noticeable difference between T6600 and T6400 - maybe a few % in doing cpu demanding tasks like unraring, encrypting etc etc.
The Acer graphics is not great but it does its job in average games/ some video editing. Even the newest games are playable on low settings.
Yeah it's a great deal on that Acer, especially if you don't want to pay more for the Asus I posted a few pages ago or blu-ray is important to you. I'd buy asus since personally I dislike Acer but it's a company that got better if it comes to quality. They are not on par with Toshiba/Dell/Hp (which I consider mid-range if it comes to build quality) but they are not far. They're fairly good and cheaper.
Also it's hard to generalize since even the worst company might have a successful, well made model.
Maybe I'm biased since I'm used to IBM laptops and love how trouble free they are.
I have my oldest one - 5 year old and use it for hours every day, take it to uni, I used to drop it numerous times (it's shock resistant) and never had a single problem with it. The centrino platform with Pentium M processor is still on par with modern celerons in cheaper laptops and the device itself doesn't look old.