Need more info besides Corsair Vengeance, what model number is it? Is it listed on your boards compatible memory list? I only see two models of Corsair, 2GB 1600 DIMMs listed.
Did you try clearing the BIOS/CMOS after putting in the new memory?
Some memory manufactures insist that all the DIMMs must be matched, even of the same model. Meaning mixing two, two DIMM kits might not work. I've gotten away with mixing two kits like you are doing, but supposedly that might not work.
If you used slots B1 and B2 previously (or A1 and A2), you were running in single channel memory mode. Usually with two DIMMs you would use slots A2 and B2, with one DIMM in each channel. Did you ever try your memory that way, in dual channel mode, with only two DIMMs?