An Athlon 64 notebook will be powerful, but heavy and battery draining. A Thinkpad is a great choice for a mobile businessman. However, the T43's are debuting right around the corner and I would recommend one of those over a T42 if you have the money. For mobility, I personally recommend the 14" SXGA+ screen and the 9-cell battery is a must for 6-7 hours of battery life. Unless he's doing any kind of 3D work (whether gaming or otherwise

) I would go with the basic X300 PCI-E video card. Do stay away from the integrated Intel graphics, because those use your system memory and slow down the system bus even with 2D use. These notebooks support up to 2GB RAM and use the newer DDR2. I would recommend either getting the system with 512MB and adding your own 512MB or purchasing a system with 1GB installed. Why does your boss need 1.5GB? 1GB should be MORE than sufficient for anyone's usage unless he has specific apps that chew up RAM.
Any of the Pentium M CPU options will suffice, but it all depends on his price range. Anything 1.7GHz or above will be more than fast enough to run any applications he needs. IBM does offer 80GB 5400RPM drives, although the DVDRW drives run a hefty premium.
He will also appreciate the integrated Thumbprint scanner that most Thinkpads now come with. You use your thumbprint to access not only the laptop but any password-protected websites or files that you program into the software. IBM support is also unbeatable. They have ENGLISH SPEAKING call centers, the main one for the USA is located in Georgia. Their repair options are excellent also. You can get on-site repair for an extra fee, or stick with the depot service. With depot service, their goal for turn-around time is that they ship it back to you the same day they receive it. :Q
I hope this helps and please feel free to write back with any other question.
EDIT I spoke with IBM last Friday and support & quality control will stay top-knotch despite the forthcoming purchase of IBM's PC Division by Chinese company Lenovo. They have a 5-year contract where IBM will basically be overseeing the entire thing, with rights to continue to use the Thinkpad name. The plan is actually for all of the PC Division folks to transfer over to Lenovo and ensure that the Thinkpad brand is protected.
P.S. As soon as a T43 is available, look for an in-depth review at the website in my sig
