Tough to answer without knowing what you do. If your dissertation involves machine learning, neuroimaging or other extremely computationally intensive work (and you don't have access to a cluster) you need something very different than if you are just going to run regressions.
If the latter, I'd just get something cheap and spend the savings on a docking station and extra monitors. Outside a few select areas (i.e. those where processing time for analyses is scaled to hours/days and not milliseconds), extra monitor space will do 10x more for your productivity than a few extra gigs of RAM that cuts your processing time on a regression from 1.5 seconds to 1.3 seconds. I have a laptop with me at all times, but its mostly for email, lighter work, outlining, etc. If I'm analyzing data or doing serious writing I have (minimum) 2 monitors in front of me so I can have my manuscript, 2-3 PDFs and/or stats stuff open at once. I guess its common now, but I have no idea how people survive with just a laptop. Storage is so cheap these days as to be irrelevant - I can't imagine a dissertation where it would be an issue. Comfort matters. I hate laptop trackpads if I'm doing anything serious.
I might modify the above if you have a grant or something and don't have to pay for it yourself. I have a Mac now. I didn't pay for it out-of-pocket or I probably wouldn't have one. Wouldn't have dreamed of it as a grad student. I'm a cheap bastard thrifty and price-conscious individual though.
