Answer #1-no you can't use the uplink port if you're already using LAN port #1; the two jacks are connected together, with the uplink port simply wired "backwards" to keep you from needing a crossover cable.
Answer #2-no, an AirPort card won't work with 802.11g. Sorry. The modulation and frequency usage is different.
Suggestion: Instead of buying a WAP11 or similar product, buy a BEFW11S4. The price difference isn't much, the BEFW11S4 has BOTH a 4 port switch and a wireless access point, and you can even split up the load, using both the switch in the BEFW and your current BEFSR41 (I have one of the original version and it's still going strong) and connecting one to the other. (You wind up with six usable ports (one from each is dedicated to the cable connecting them), wireless networking, and not too much out of pocket expense.)