Memory cards are not region locked but US and JPN saves can not co-exist on the same card. The only change I can think of is that the component output was removed from later consoles in both regions, if you are a videophile you may want to confirm that you are buying an early console
I'm guessing you already knew this but if you are experienced at soldering, you can buy a NTSC-U console and mod it. I believe the compatibility is perfect so there's no advantage having a japanese unit instead
http://www.gamesx.com/importmod/ngcmod.htm