It likely does not matter if they're the same speed.  That said, unless you're doing unusually memory intensive work, a single 16 will result in better performance than using 24G in 16+8 -- if you do, only the lower 16G will run in dual-channel mode;  the rest will be in single-channel.

Thanks, why would adding more memory decrease performance? If I was to use the 16 only it would be single channel mode also. Just curious.

Because for full dual channel both memory sticks must be of the same brand.