I think it's safe to say that within the margins of SB bClk overclocking (5% ish) only System Agent and memory related voltages matters.
Typically your RAM will run a couple of cycles faster (unless you compensated for this by lowering the multiplier/increasing divider) and that means your memory controller will also run faster. Actually the entire "uncore" (parts of the CPU that are not used for calculations/cores) will run faster. The "uncore" is called System Agent.
The PCI-E controller however will also run faster, which will be the bottleneck when you overclock via bClk.