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.
Thank you very much for posting Paul.
We understand that you have some questions on regards to the Intel® PRO/Wireless 3945ABG Network Connection.
On regards to your question please note that this adapter is meant to be professionally installed by Original Equipment Manufacturer (OEM) technicians on certified systems only also the adapter is not meant to be sold separately.
In case if you want to replace the card for a newer model we recommend you contacting your Original Equipment Manufacturer (OEM) to determine which product is approved for use in your computer.
For additional information, refer to:
Intel® WiFi Products - Upgrade or exchange Intel® wireless adapter hardware
Intel® WiFi Products - Why doesn't my laptop recognize my new Intel wireless adapter?
I hope this helps