I know it's been a few months since this discussion but I was wondering the same thing and just came across some information that might be of interest.
Apparently the difference is that the SLGQ8 part supports VT-x but the SLB9Y part does not.
Still no conclusive info about the cache, but the VT-x thing would account for the different part code so perhaps they're the same apart from that.
SLB9Y E7400 2.8GHz (10.5x multiplier) 45nm, 1066FSB, step R0, 3MB L2 no VTx
SLGQ8 E7400 2.8GHz (10.5x multiplier) 45nm, 1066FSB, step R0, 3MB L2 with VTx