This link might be useful:
http://users.erols.com/chare/video.htmThere's technical data on nearly all the 3d chipsets out there, as well as his own subjective ranking of nearly all the chipsets.
You can then check out online stores as to what the prices are. I don't think there's that much selection for AGP cards these days, especially one that is acceptable with ZT2 and in your price range, so that should make your life easier.
I use an ATI-built 9600XT AGP 128MB with a P4 2.8G on a FSB800 w/1G DDR400. Running 1024x768x32 and assuming a fully packed and populated medium size zoo (about 120 guests and 120 animals), I have to use Medium detail to have acceptable frame rates. Definitely no anti-aliasing. Also, "acceptable frame rate" is a relative term. I'm talking about 10 to 20fps assuming a typical line of sight, definitely not playable in terms of first-person shooters or racing sims, but ok for this game. If I take a line of sight that has a very long and broad view of a medium size and fully packed zoo (like standing on a hill or bridge overlooking the entire zoo), it goes down to like 4fps. I haven't built a fully packed large zoo yet, so I don't know how bad that's going to be (the processor might start to limit as well). If I use anti-aliasing and High detail level, the game will always crash within a couple of minutes of walking around in a medium sized fully packed zoo, not to mention the frame rate is down to like 1/4fps.
My recommendation is to definitely get something equal or better than a ATI-built 9600XT if you want to enjoy the game with at least medium detail.