So what I need to find is an inexpensive ($100 or less) card that is comparable to the 4200, or slightly better, to get her through until I can build my next system is 6-9 months. Any thoughts or recommendations?
Have you already checked with your GF4's manufacturer about your warranty? Never know, might still be able to get it replaced.
Anyway, for ~$100, you can get...well, not much worthwhile, frankly.
The Radeon 9600 Pro is probably your best bet: you can find one for just over $100. A GeForce 5700 is probably an acceptable alternative, but it is a bit slower. Unfortunately, ATI and nVidia's newer budget-minded cards (namely the X600 Pro & 6600) aren't currently available in AGP form, AFAIK.
Try scanning eBay for a good deal on a used card. I see some 9700 Pros for ~$100, which are substantially faster than the 9600 Pros. [Heck, it's a little faster than your 5900XT in some cases.]
Of course, you can also just buy a total cheap-ass sub-$50 card to make do with until you upgrade your PC. Depends on how demanding of a gamer your wife is...and how much you value your marriage.
Or you could just use this as an excuse to buy a better AGP card for yourself and postpone your PCIe upgrade. Hey, we won't rat on ya...
BTW, a handy reference for comparing AGP cards is here:http://graphics.tomshardware.com/graphic/20041004/index.html