The list above applies to the IR RDC-7.1.
Heres what is included as standard for each model:
RDC-7.1 - Analogue Audio, Digital Audio & I-Link Modules
DTR-10.5 - Analogue Audio, Digital Audio, AMFM & Base Video Modules
TX-NR1000 - Analogue Audio, Digital Audio, AMFM, I-Link, Net Tune, Double Multichannel Input, RCA Component, HDMI & Base Video Modules
What is worth noting is the Onkyo is NOT customisable like the other two, all slots are full. What will most likely be available to Onkyo users is an upgrade to the next revision. For example if the HDMI 1.1 modules is released then Onkyo will likely offer the NR1000 with it and probably offer an upgrade for existing users (Swap old HDMI 1.0 card for new 1.1 card).
As you can probably see the NR1000 is the value for money option but the DTR-10.5 can offer things like AES/EBU and BNC component connectors if the user requires them. There are also some small tweaks applies to the DTR-10.5.
What can go in what slot seems to be limited more by the firmware than hardware. At first I thought only similar cards can use the same slot (Eg. Only Component BNC or RCA in one of the slots) but in Japan the RDC-7.1 can take either AMFM or D4 module in same slots (These modules are not at all similar). Numerous items is so far not possible probably due to firmware restrictions.