From other readings in these forums, it does seem that mixing wireless "g" with "n" and expecting smooth fallback isn't realistic. You might try to temporarily lock your wireless on "g" to see if the complaints stop. I believe you can use windows "tracert" to determine the connection path of your wife's PC just to make sure she's connecting through the desired device. As for network finders/sniffers, etc., I believe such as "netstumbler" might assist in determining who else is broadcasting locally, at what signal strength, and what channels are in use.
I have recently replaced my old Wi Fi routers. One was actually running as a router, the other as an access point in another part of the house, with a Cat 5 connection between the two. I couldn't decide what to buy so I ended up getting a Belkin Wireless N and a D-Link DIR-655. Both have had some good reviews.
The D-Link is functioning as a wireless router, the Belkin is configured to be a wireless access point. They are on non-overlapping channels (D-Link is on Channel 11 primary, paired with Channel 6 secondary and Belkin is on Channel 1).
My wife complains that her laptop (Vista Home Premium, 802.11 b or g capable) feels sluggish since the change. The other PCs, either wired or wireless, have not felt different to me, and I don't use her laptop enough to have an opinion. Most of the time she is logged on to the Belkin access point and is the only device attached to it. My laptop (Win 7, 802.11 b, g or n capable) connects to the D-Link. It is possible that she is encountering interference from a neighbor or it is possible that one of the wireless devices is functioning below specs or other problems may be coming up. Unfortunately I no longer have the old WiFi access device she used up until recently because I gave it to my Father-in-Law so I can't do direct measured comparisons of speed and I cannot easily check which channel the old one operated on.
This raises several questions:
Is there any good way to determine whether the apparent sluggishness comes from things outside our house, the access point, the router, her computer, or a source between keyboard and chair?
Is it possible to determine which wireless protocol she is actually using (as opposed to which protocols are available)?
Is there any easy way to figure out what channel the neighbor's WiFi is using to see if interference seems likely?