buy a notebook

Most HPs with Intel will run out of the box (try 'Lenny' rather than 'Sarge').
If you're daring, install a base system, upgrade to 'Sid', then install the rest of the system.

I can't speak for all Intel HPs - look at the ones you want (and can afford) and write down the supporting chips and check that they all have drivers for Linux. Well, with the exception of the WinModem - unless you really want a WinModem.

The AMD-based HPs often have supporting chips which cause some annoyances with installation, so in general I do not recommend them (even though I love AMD and have a still-awesome Opteron desktop).

As for DELL - do as with HP - list the support chips and make sure they have native Linux support so you can do your best to avoid fiddling and avoid installation of tools like ndiswrapper. The only exception I make when I do my shopping is for the graphics chip - I'll accept an NVidia graphics chip and tinker to get it running. ATI (now AMD) is also improving support, so I may consider that as well.

when ever I get into buying any new hardware .. I go to http://uk.ezilon.com/cgi-bin/jump_uk/jump_search.cgi?cat=1&q=hardware&x=0&y=0 surly I search only on UK companies ... you can rely on them