The ASUS has an older 65nm CPU and only uses DDR2 RAM. The other two have 45nm cpu's though neither support virtualisation. They will at least have a newer chipset since they have DDR3 RAM. So I would personally discount the cheap ASUS.
If the machine is going to do a lot of travelling get the Lenovo, their reputation for build quality is second to none, otherwise go for the cheaper Acer and get the bigger HD.
I'd like to know the screen resolution though, the Lenovo is WXGA, which is normally 1280x800, but on current 16 x 9 screens may well be 1366 x 768. The vertical resolution of laptops is getting smaller all the time, this means less real estate to work on and more scrolling to read web pages!