MacBook Pro Airport card upgrade?

I have an early 2008 MacBook Pro, the very last of the non-unibody models. I want to upgrade the airport card. My wife's black Macbook (which is even older than mine) gets a stronger signal from our Airport Express (and other) wireless modems, and she can use her machine in rooms of the house where I get no reception. I am running the latest version of Lion and have done all software updates for Airport.

I have read some things online stating that these older Pros have relatively weak airport reception. But I can find no recent information on a good card upgrade option. I can do the spanner work myself (I have already upgraded RAM & HD in this machine a while ago without problems), but just need to know what to buy, if there is a better option. I did find some info stating the card out of the Mac Pro was a suitable upgrade, but this info is a few years old now, and also gave no further info. Is there anything more recent? If not, where can I get a Mac Pro card? Apple store, or is it a generic part I can get elsewhere? Can anybody steer me towards a product or part number, either Apple or third party?

I just want to get a couple more years out of this machine before I upgrade, as it still does everything I need it to. A new machine would be nice, but a hundred dollars or so making this one a bit more flexible in my house would do me for a while.

Is that because there is nothing available to upgrade? Why is the performance better on my wife's older, lower spec model? This was no concern to me in my small Tokyo apartment, but since we moved to a bigger house my wife's MacBook works in every room, whilst mine doesn't.

We're using the same Airport Express base station as in our old apartment, but I suspect the difference there was just the apartment was small enough it never became an issue. It was also an issue in a couple of other places we stayed in the interim, using other wireless systems.

Anyone else have any suggestions?