Despite the lack of native 64-bit apps (they are emerging however, have a look
here and there's even a 64-bit version of the game Far Cry), 64-Bit processors are the way to go at the moment. The Intel and AMD medium/high end processors all include 64-Bit technology (except presumably the Pentium-M), and even without 64-bit software, they're still at the cutting edge of 32-bit performance as well. Whilst not supported by all hardware manufacturers yet, and stuffy about some legacy software, the 64-Bit edition of Windows XP actually offers a performance increase in many applications, even if it is just because the operating system behind those apps is running more efficiently, rather than the applications themselves. If you are faced with an AMD64 laptop over an Intel Pentium based system, check to see if it's a Turion based system or just a desktop Athlon 64 crammed into a laptop shell. Battery life and mobility will be greatly increased with a Turion based system, and those "extra bits" will give you a bit of forwards compatibility with the likes of Longhorn (sorry, I mean "Vista"
). Similiarly, if battery life and connectivity are important on the Intel side, you'll want a Centrino/Pentium-M system, but I'm not sure if these feature 64-Bit yet.
Itanium was a flop with regards to 32-bit applications, but the same cannot be said of the Athlon 64 lines or the new Intel EMT64 lines. The fastest 32bit systems nowadays are 64-bit anyways, so there's really no reason not to have one.