A 'bus' is simply electrical pathways that by way of 'hubs' (the north bridge and south bridge) connect the various components of the motherboard.
Most importantly: Hyper Transport (HT) or Quick Path (QP) 'links' (or 'interconnects') replace the front- (and back-side) 'bus'.
The 'speed' (actually, 'transfer rate') of the 'bus' depends upon the width of the path (8-bit, 16-bit, 32-bit, etc) times the speed (frequency) of the bus (33Mhz, 66MHz, 100MHz, 133MHz, etc) times the transfers per clock cycle (... double- or quad-pump).
With HT and QP (I assume - Intel is still a little pricey for me) the speed of your RAMs is dependent upon the integrated memory controller on the CPU.