The make and model board used there as well as the two sets of memory you are talking about would be a help. An older board like in some prebuilds may only take single sided valure memory and won't run dual sided dimms.
That's dimms seeinhg a row of small ic chips on both sides rather then just one. The single sided memory also leaned over a certain amount while all dual sided sits straight up.
The first thing to look at is the specifications for the board itself whenever plaaning to add more memory or replace the existing to see just what is supported as far type along with speed. Often you mix DDR266 PC2100 memory with DDR400 PC3200 to see the faster memory slowed down to the PC2100 speed.