First of all, the PCI interface on the new HD (Core and Process) cards is much more powerful than the one on the MIX cards. This is the primary reason (along with more sophisticated DSP code) that enables us to now support 128 tracks at 48 kHz while the PCI meter rarely goes above 60% (-6042 errors should be very rare for most people now). The PCI interface on the HD card supports both 32-bit and 64-bit PCI busses, which means that the older 32-bit chassis configs should work.Will anyone see a difference between a 64-bit chassis and a 32-bit chassis? Well, for "basic" Pro Tools rigs (no video, Unity, etc.), probably not. However, in the next release of Pro Tools we'll be supporting the AV and Unity options, and on these configs you'll probably see a difference (since more PCI traffic is involved). 64-bit PCI interfaces allow the HD cards to burst data more efficiently.Therefore, an HD card *will* work in a 32-bit slot (whether it's a 32-bit slot in the main CPU, or a 32-bit chassis).The reason that we've qualified only the latest generations of G4 machines (with AGP video cards) doesn't really have much to do with the 64-bit interfaces per se. However, we've noticed better PCI performance on the last few generations of machines, and in particular it helps to have the video traffic (i.e. from the computer monitor) go across AGP rather than PCI. This is the main reason we only qualified AGP-enabled G4 machines.Does this mean older G4 machines (non-AGP) or G3 machines won't work with HD? Nope. It just means we didn't spend time qualifying them (so they're not officially supported), and if you use an older computer the max track counts won't be guaranteed ("...your mileage may vary...").In general, if you have a computer that runs well with a MIX system, there's a good chance that an HD system will work even better (since the PCI interface on HD is so much better). For example:If I had an existing MIX setup with an older computer/chassis and wanted to upgrade to HD, I wouldn't run out and buy a new computer or chassis right away. I'd buy the HD system and try it out with my current computer/chassis, and only upgrade if I ran into performance/compatibility problems.