You may well have a 133GB hd drive limit on the old PC. This will depend on your motherboard and not the OS I believe, but it may depend on both. Partitioning will not help get over this limit. What mb do you have ?
centrino 1.4 is about 1/2 the speed of pentium4 2.8 (or AMD) but will give about twice the battery life (and less noise and heat). I doubt you would notice much difference in speed in 'normal' apps like web,email excell etc. You would notice it if you were converting an mpeg2 file to divx or similar heavy CPU intensive app. Have a look at
www.tomshardware.com for comparisions of various mobile chip options.
HTH