Hi,
I actually tried to change the 3945ABG for an Ultimate-N 6300 on my HP NC2400 laptop and at first it didn't work, due to a BIOS whitelist that specifies what cards the computer should be able to boot with. Seems to me like it's something mostly HP and IBM are using, and in my opinion it's really stupid, restricting the possibilities for their customers to upgrade.
Happily, there is a work around - flashing the BIOS to one without the whitelist. Take a look at this link for an extensive list of downloadable whitelist free BIOSes:
I got my card of of Ebay and I'm only using two antennas (don't need bluetooth, so I simple connected that antenna to the Ultimate-N 6300 as well), but still it reaches three times the speed of my 3945 - best so far is a transfer in ~58Mbs.
Recently got a third antenna that I will install when I find the time.
I hope this will be useful information for someone out there, if not for you Paul.