Most memory is downward compatible.. But you want to make sure you're getting DDR1 instead of DDR2.. Also, some motherboards are picky.. gigabyte boards are known for this, get quality ram.
if the mobo spec is PC3200, thats mean maximum can support up to PC3200 speed of ram only.
If u want to try PC4200, thats mean overclock. Whether can overclock or not have to depends on how strong is your mobo ~~~