Stay away from centrino for the time being. Any internal wireless cards have been very bad at receiving signals. my sony and my latest IBM are centrino and i still use a pcmcia card for the wireless connection instead of the onboard wireless adapter. although it is a great idea, i wouldnt count on it for at least another 6 months-1 year.
Have you only used the crappy intel wireless adapter as far as internals go. I assume you have, since it was a full "centrino" instead of pentium m. I'm just curious, becuase I've ehard that the intel wireless chip is horrible, but that in general mini pci adapters are better in every way than pcmia cards. . .
g is the fastest, most expensive, and least common.
An X30 or X40? I don't even know if the X40 is available outside japan yet, but I don't really follow notebooks that closely. By all accounts, the X series are wonderful machines, and you can't go wrong with one. Just realize than on exam days you may have to remember to bring your media slice with you, for the cdrw/dvd drive.