A word of warning - not all AGP cards fit all motherboards. You need to know what speed you mobo supports and get a card which fits. For example, your MSI 6340 motherboard supports 4X so you need a 4X or better AGP card to fit. I think all your choices will fit. Next consider memory - DDR is faster so better, but frankly, I would prefer to spend a little bit more for 128 MB. Ebay can come up with some bargains, if you're prepared to wait a bit. Don't forget to disable the on-board graphics, or your new card won't be seen.
So far as your son'e and your PC's are concerned, can't comment as you don't mention your mobos.