Javintosh: your AGP monitor will see the boost. The PCI will not see the boost. Kinda like on my machine: I've got a PCI Rage 128 and a PCI Radeon 7000. If I kick on the visualizer in iTunes and drag it to the Rage 128 monitor, the frame rate goes down. If I drag the whole window to the monitor connected to the Radeon 7000, the frame rate jumps up. If I put the visualizer halfway on the Radeon monitor and halfway on the Rage 128 monitor, I get the lowest common denominator (the Rage 128 framerate) and sometimes slower.
Everyone else: I hope I can clarify this topic somewhat by what I'm going to say.
Jaguar (aka 10.2) will be faster than 10.1 in many respects. One will be graphics performance. EVERYONE will see a boost in graphics performance, no matter what card you're using. This is due to better code and what-not deep within the bowels of the system. No, it won't be lightning-quick, but simply faster overall.
Now on to Quartz Extreme: EVERYONE with an AGP-based Macintosh with ANY Apple-supplied AGP NVidia card will see the benefits of Quartz Extreme on the monitor connected to the AGP card. EVERYONE with an AGP ATi Radeon card, be it pre-installed or aftermarket WILL see the benefits of Quartz Extreme. Simply put, an AGP NVidia or AGP Radeon card IS THE ONLY REQUIREMENT OF QUARTZ EXTREME, regardless of how much video memory you have.
Now, 32MB of video RAM is what Apple recommends to see the full potential of Quartz Extreme. Got a 16MB AGP Radeon card? It's ok, you'll be able to see Quartz Extreme -- just not as well as someone with a 32MB AGP Radeon card.
Got a PCI Radeon 7000 card? You're out of luck. You've got a Radeon, yes. You've got 32MB of video RAM, yes. You've got AGP, no. AGP is required.
Apple never made a motherboard with 1x AGP. Everyone who has an AGP Mac has at least 2x AGP.
Another thing is that Quartz Extreme support is NOT required to RUN Jaguar. Jaguar will run on every piece of machinery that 10.1 runs on. Quartz Extreme is kinda like a modem in a respect -- will your computer run without it? Yup. Will you be missing out on something? Yup. Will it make Jaguar unusable on your machine? Nope.
Recap: Quartz Extreme requires an AGP NVidia or AGP Radeon card. Whatever video RAM. That's it. Plain and simple. The rest of the specs are recommendations (not requirements) to get the full experience out of Quartz Extreme.