First off I think that 4.5MT/s is a typo and it should be GT/s, the X58 FSB is designed to run at either 4.8GT/s or 6.2GT/s. I've never heard of a speed that low before.
The "FSB" refers to the Intel Quick Path Interconnect that connects the CPU to the IO Controller Hub. On Xeon multi-socket motherboards it is also responsible for connecting multiple CPUs. On the desktop boards the main job that the QPI link performs is transferring data from the memory controller located on the CPU to the PCIe controller on the IO Controller Hub where it is then sent to the video cards, SATA controller, etc.
My understanding is that a faster QPI (FSB) isn't going to affect performance much, if at all. Higher QPI speed's are going to help in situations where you have a multi-socket motherboard with multiple CPUs.