and model number.
Based on the limited info provided it sounds like you're talking about DDR and DDR2...and they do (usually) not work in the same system.
hello...i have one stick of 512 in my comp right now and i want to put a second one in...there is another slot for it..the one i have in right now is a PC3200U-30330 512MB DDR 400MHz CL3 i have been in posession of a couple other 512MB sticks but are PC2-4200U-444....can i just add one, or put the 2 i got into my comp? if i put them in would it mess my comp up? XD