Internal modem's PCB connectors comes in two sizes, if its about 5.25in(13.5cm) then its an old(hardly seen anymore) ISA type modem and should be placed on the long black slot, if its short about 3in(80cm) then it a PCI type modem and should be placed on the short white slot. In either of the two, which ever is free, in no particular order, you can safely insert these modem and just make sure it is properly seated.
On an AT motherboard, the CPU will be placed in front of the expansion slots
Its not mini PCI, its just PCI, they just look mini compared to a PCI-E. It depends on which PCI slot it is, there are tons of different cards that are for PCI. Sound, wireless, modem. Look at the spec's for the motherboard to see what slots you have, then you know what cards you can use.
Memory slots
You can use USB adapters. But USB does not cover as many interfaces as expension slots.
No, Memory slots are on the motherboard bus.
You cannot actually insert pci slots into a PC since pci slots come inbuilt on the motherboard. so the no. of pci slots u have depends on the motherboard.
Find the use's manual for the motherboard and you will find all required information.
A motherboard
It depends on your motherboard. Some motherboards have only 2 slots while others can have up to 6.
If you want to change the RAM slots, you need to change the motherboard.
Memory Slots
The motherboard.