What's better: dual-channel memory mode or a bit more RAM?

[Adam Funk]

I had 2 × 512 MB RAM in my home computer and a colleague offered me 3
extra DRAMs that he had at home.  My motherboard has 4 slots and
supports Dual-Channel Memory Mode if you put 2 or 4 more or less
identical cards in.  I have two choices.

A. 2560 MB of RAM in Single-Channel Memory Mode (according to the
   BIOS), consisting of 3 × 512 MB + 1 × 1024 MB.

B. 2048 MB of RAM in Dual-Channel Memory Mode, consisting of 4 × 512
   GB.  The motherboard manual seems to indicate that all 4 cards must
   be exactly identical for Dual-Channel to work, but the BIOS says
   it's in Dual-Channel even though there are 2 pairs of identical
   cards that are not identical to each other (different manufacturers
   and speeds --- the system seems to treat all 4 as having the speed
   of the lower 2).


Will A or B give me better performance?

Thanks.