Joaquin,
For B250 blade with two CPUs, we must populate both CPU's memory channels in identical fashion. In your current configuration, another pair of 4 GB should be added to the F memory channel to even out the memory arrangement ( total of 96 Gb / 12 pairs of 4 GB ).
Also, if the new total system memory capacity requirement is 128 / 160 / 192 GB, then it should be evenly spread across two CPUs by following below configuration rules.
In B250 DIMMs are installed in pair
Within a DIMM pair, we need same DRAM vendor, REGISTER vendor (Inphi/IDT), density (4GB/8GB/etc), and organization (x4/x8/x16).
Within a channel, we need same density (4GB/8GB/etc) and organization (x4/x8/x16), but DRAM and/or REGISTER vendor can vary.
If you are planning for 8 GB DIMMs, they should be installed in separate memory channel of each CPU.
Please refer to following guide for more detailed information and memory installation requirements.
http://www.cisco.com/en/US/docs/unified_computing/ucs/hw/chassis/install/fullblade.html#wp43041
HTH
Padma