My first comment is that any laptop that came with an ABG wireless card is simply too old to make this upgrade worth the effort.
My second comment is that, in many cases, these old laptops used inclusion and exclusion lists in the BIOS to prevent the user from upgrading the wireless card themselves. I usually recommend that you check with the laptop vendor to find out if this is the case.
The Pro/Wireless 3945abg card utilizes the original Mini PCIe connector. The recommended replacement for this card would be the Wireless-AC 7265. The version of this card that you want to look for is the 7265HMW; this is the one that is designed for the Mini PCIe connector. You specifically do NOT want the 7265NGW; this is designed for the M.2 connector, which you do not have.
Oops, almost forgot: your existing card is a full-length Mini PCIe card while the replacement is a half-length Mini PCIe card. If the laptop has the screw hole for the shorter cards, great. If not, you may need an extender bracket to make this work. Here is an example of what this extender bracket looks like: https://www.amazon.com/Pocaton-Express-Bracket-Adapter-Wireless/dp/B072R2NQQS (yes, it's just a piece of metal to make the card longer).
Hope this helps,
...S
