Adding memory of the same type probably is possible. Check the specs of your motherboard.
Replacing the memory by another type probably isn't possible. Again, check the specs of your motherboard. You didn't tell what you have, so we can't do it for you.
For your second experiment, it seems you need another bus. Buses are hardware on the motherboard. So you might need another motherboard. But that changes much more than just the bus, so the comparison isn't fair.
Your third question on bar code readers:
- between the keyboard and the PS/2 slot
- serial interface
I want to upgrade my PC's memory capabilities. The system has one bank of PC-3200 DIMMs installed. I want to upgrade the system to have two banks of PC2-6400 DIMMs. What actions do I need to take to install these memory modules?
I want to compare the data transfer performance of the PCI local bus using a 32-bit bus versus a 64-bit bus running at the same speed.
Can you describe the different methods commonly used to interface bar code readers to a PC system?