I'd expect something along the lines of this, though of course this is just my speculation.
Low 21.5"
2.8GHz i5 (3450S)
4GB RAM
1TB HD
AMD 7750m 512MB
High 21.5"
3.0GHz i5 (3550S) - 3.1GHz i7 (3770S) Upgrade
4GB RAM
2TB HD
AMD 7770m 1GB
Low 27"
3.0GHz i5 (3550S)
4GB RAM
2TB HD
AMD 7770m 1GB
High 27"
3.3GHz i5 (3550) - 3.4GHz i7 (3770) Upgrade
4GB RAM
2TB HD
AMD 7970m 2GB
All hard drives upgradeable to 3TB and a 256GB SSD additional.
RAM upgradable to 16GB either running at 1333MHz or 1600MHz, 8GB RAM could be standard in the high end model but we will have to wait and see, most people are probably fine with 4GB.
USB ports are now all 3.0 along with all the ports it currently has now.
Graphics cards and VRAM could be NVIDIA Cards with the likes of 660m and 680m with between 512MB and 2GB of VRAM.
As for the optical drive the iMac may or may not keep it.
I think an i7 will be standard for all the iMacs apart from maybe the very low end one. That's what they did with the latest MacBook Pros.