Ok, you only have the 16mb version? You can upgrade the unit to 32mb with a standard 72-pin SIMM memory module. You'll have to upgrade the BIOS if you do (it's done by a software upgrade).
And if the game you want to play is 32mb large it should work, no matter if it's a PAL or NTSC game cartridge (unless it's one of those games that are protected or larger then 32mb).
The Z64 doesn't care if it's a PAL or NTSC game inserted in it.