As I sit here typing away this update, I have installed the Linksys a+g (wpc55ag) card into my pcmcia slot. In the exact same spot that I got a low signal when I was using the internal card, I'm now getting a very good signal. So it's about a 100% improvement in signal strength. Through out the whole house I get no less than a very good signal. The furthest point I'm away from the router is about 30 feet and two rooms. So my wireless network is finally set up with a Microsoft MN-500 router and the Linksys external card, and it work well for me. I like the Microsoft router better than the Linksys G router I had for two days. The Microsoft router seems to give me a little better signal strength.
Now the above being said, I'm still not happy about the situation with the internal card. Given a choice, I would prefer to use the internal card only instead of the external one (less crap to carry around and more stuff to lose). But like other's have said, until we or AW finds out what really is the problem with the internal card, it's best to hold off buying an internal card from another vendor. If it's an antenna issue or case design issue a different internal card may not work any better. For now, I'm happy with the Linksys A+G card.