You should use the screen at its native (i.e. max) resolution. While 1400x1050 on 14.1" might be too small for you, you might not mind a 15.0" T60's 1400x1050, although the machine would probably be too heavy and bulky, and that particular LCD was notorious for being dim. I recommend the T61 with XGA and Intel graphics if you must stick with XGA, but if you don't mind widescreen, the T400 with 14.1" 1280x800 is worth considering. Things will look a bit smaller than on the 600X's 13.3" XGA, but I bet it would still be tolerable.
Dell Inspiron 7500; Gateway NX860X; HP Pavilion x2, EliteBook 8740w; Panasonic CF-Y9; Sony Pro13
Dell OptiPlex 9010,790, Precision 390; HP d7900, Elite 8300,8200,8000
Crossover 404K; Dell 3008WFP,P2815Q,U2711,sp2309w,2007FP; HP ZR2740w; Lenovo L220X; LG 25UM65; NEC LCD2070NX; Samsung 2343BWX