Website vs In-Store

MT6840: 15.4'', CoreDuo T2450 (2.0 ghz), 1gb RAM, 160gb HDD, a/b/g wireless and Vista Home Premium

MT6459: 15.4'' Turion X2 TL-56 (1.8 ghz), 2gb RAM, 160gb HDD, g wireless and Vista Home Premium

MT6841: 5.4'', CoreDuo T2450 (2.0 ghz), 2gb RAM, 160gb HDD, a/b/g wireless and Vista Home Premium

All of those use integrated graphics so don't expect to play many games on them.

IMO it is better to configure them through the website so you can get the system that best matches your needs.
Take a look at the Dell Inspiron 1520 and the HP DV6500. I don't know much about Lenovo's products so I'm no help their except that the have good build quality.

Have you checked your school's website for any arrangements with one of these manufacturers? Some do, and that makes all the diff.
Online is a good way to go, even if you don't want a discrete graphics card. That way you can make sure you get a DVD burner, or 802.11 draft n. Hard to say if you'll need that stuff, but it could give your pc some longevity.

As for the Dell 1501 vs. 1520, thats AMD/ATI vs. Intel. The Intel CPU will be superior on the 1520, but I believe the graphics on the 1501 (Radeon xpress1270) are better. For pretty casual usage, may not even be noticed whichever way you go.
As for the others, I dunno. Guess you have to decide