Jim, I'll weigh in here also on various points.
1) Most people in your position would just get a linksys type wireless and replace their wired, but consideriing you want easy on/off with a mechanical switch (versus a software switch) then it's probably best to "hang" a wireless AP (or router) off of your wired router.
To turn it on and off, pull the plug from the back of it. Or clamp one of those inline lamp switches onto it's power cord. Or plug it into it's own power strip with a switch.
You do not strike me as the kind of user who wants to get involved with DD-WRT or other alternative firmwares at this point, so give that a pass - for now. BUT, if you do think you might want to someday, then maybe you should get a compatible router for that. A linksys WRT54GL is good for that - they cost more than the regular cheapo Linksys 54G. The basic model has been downgraded over the years and the linksys WRT54G (no L) you can get at Walmart is weak and no longer has replaceable antennas. It's ok for basic nearby wireless, but it not very future-proof device so only buy the Walmart version if convenience and price are most important..
I suggest the WRT54GL on Newegg or even better is the Buffalo WHR-HP- G54 (at newegg) to get a quality powerful router that will serve you for any unforseen future configurations. The Buffalo has the best .coverage of any G router. Both will readily take DDWRT or Tomato if ever needed.
Linksys "L" router:
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"HP" router:
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2) You don't mention anything about the distances or coverage that you will need for your wife's pc or your kid's. This is important. If you need good coverage, then the "wounded" (no antenna connectors) version may not work. If you find that the regular antenna doesn't reach, then the better routers I mentioned will allow you to put a high gain antenna on them. In fact, that Buffalo already has a high- ish gain antenna. It's got great coverage.
3) Wireless USB adapters - I also prefer ones that have external antenna connections for the same reason. If reception with the built- in (usually weak) antenna is insufficient, then you can put something stronger on it.
Take a look at this one:
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Really, you need to give us some idea of your layout for wifi (distance, walls to pass through, construction) before choosing devices.
I agree that G is a good choice if your neighborhood is not over- crowded with wifi and you don't need to stream HD video over wireless.
Steve