I had very bad experiences with Toshiba reliability in the early 2000s and have not touched them since. While Dell reliability is poor as well, in Dell's case they make up for it with generally rock-bottom prices and generally pretty easy to deal with warranty service.

What's the warranty on each? Any accidental damage insurance included?

If the prices and coverage are all equal, the Thinkpad is a no brainer.

It's much lighter, solider construction (IMO) than either the Toshiba or the Dell, discrete video vs. integrated (which will make a bigger difference, again IMO, than the processor at that end of things) , better WLAN than the Dell.

Another question is "which battery does each come with" - more cells = more weight, but longer life. OTOH, if you plan to use it on battery a lot, moving to a "P" series rather than "T" series CPU will also help your battery life, regardless of model.

Oh, as for XP/Vista/Win7 - the generation of integrated graphics in that generation of machine should be fine for Aero.

HD is about the easiest thing to upgrade these days - given how often ram is getting harder to access, even easier than RAM in many cases. At this kind of midrange business machine, I'd be tempted to go SSD anyway, which is going to be even smaller.