It won't be less than an 8-core Xeon-W (e.g. iMac Pro) as a base configuration, and probably more than that - Apple wants this to be the fastest Mac yet, not "base configuration is slower than a glorified iMac". By the time they get it out the door, there may well be an 8-core
consumer iMac, which would move the iMac Pro to "10 cores and beyond" or even "12 cores and beyond". At a bare minimum, the base Mac Pro will be as fast and core-laden as the base iMac Pro, and I'd actually expect it to start a step faster. I expect it to use a "big-socket' Xeon to accommodate the 28-core option at the top end, and possibly dual processor options, although the lesser options will probably be essentially Xeon-W chips in the bigger socket.
Similarly, I wouldn't expect any less than 1TB of SSD connected through the T2 (T3?) as a minimum option. If I had to guess, the configurations will be 2 TB, 4 TB and 8 TB. I am certainly hoping there are standard M2 SSD slots in addition to the proprietary storage going through the T2 (I would
not expect MacOS to boot off of anything except the proprietary storage).
Depending on whether the chips have 4 (standard Xeon-W) or 6 (standard on big-socket Xeons) memory channels, the base RAM will probably be either 32 or 48 GB, using 8 GB DIMMs and filling all channels. I'd hope for 2 slots per channel, which would give a maximum RAM capacity of 256 GB or 384 GB using standard 32 GB DIMMs, with the possibility of doubling that if 64 GB DIMMs were supported. Hopefully the RAM slots won't be too badly blocked by some other component - we don't want to think about what the Apple Tax on half a terabyte of RAM might be!
They
will do something to block NVidia video cards... Best case is that the video card(s) are in standard PCIe x16 slots, but they have a nonstandard power connector and an internal video-out that routes the video signal over Thunderbolt 3 - they might not even have a back plate if the only output is over TB3. That would make it worthwhile for AMD to keep making upgrade cards - the differences from a standard card are trivial, and even a run of a couple of thousand is worth it.
The middle (and probably most likely) case is that the video card is easily replaceable, but it's more nonstandard than the above. One possibility is that the PCIe connector also carries the video-out to TB3 (and/or the extra power), and isn't a standard PCIe connector, while the card is a nonstandard shape. It's probably electrically just PCIe, but there are substantial physical barriers...
The worse case is that Apple just solders the video chip on the motherboard...
If there are any fully standard PCIe slots, they'll be x4 - Apple may well have heard the complaints about music interfaces and the like, but will be dedicated to keeping NVidia cards out.
This is a powerful machine, but it's not a cheap one. ~$6499 base, with options that reach the stratosphere. I wouldn't be at all surprised to see a $20,000 configuration without putting huge amounts of money into Apple-Taxed RAM and storage. Even without getting into dual processors, the highest processor upgrade (to 28 or 32 cores, depending on what Intel has out by when the machine is released) will probably be a $5000+ option. Similarly, a $5000+ video card upgrade wouldn't surprise me. In both cases, there will be other options in between the base and the monster upgrade, of course. It will probably be possible to get the machine well over $30,000 by recklessly paying Apple Taxes. 512 GB of 2666 MHz ECCRAM is over $5000 at Newegg, and really top-end PCIe storage like Apple uses is at least $350/TB even before Apple Taxes. If they permit both dual processors and dual video cards, there's probably an insane configuration pushing $50,000 (who knows if they'll ever sell one of those).