Is it OK to charge a few (2) mp3 players via USB on the same computer at the same time? Will be put too much of a load on the circuitry of the computer?
Your PC will work with one at a time and the speed it goes will be in relation to the memory available. You may notice a time delay with a PC with low memory but it should not be a significant difference. As you probably can not physically change the two at once (unplug and plug in) your PC might even complete the first task before you can action the second.
I'm concerned about the current flow through the motherboard... would plugging in more than one USB charging device overload the circuitry in terms of current?
As I said before, the PC will only handle one job at a time so while you will be running two USB's the PC will only load one at a time. Each machine varies but it probably could manage 8 or more at a time.
No, it will charge fine. The current that goes through the board to the RAM and CPU is massively higher than the amount of current going to the USB ports.
If you are now happy and do not require any more information, could you please mark this thread as solved.