In computers 'bus' is the physical bi-directional transfer of electronic information. So increasing it will increase the rate at which information transfers between your cpu and other components. You may be wondering why it isn't already set at 150mhz. Well the increased clock speed as measured in cycles per second, (hertz, or mega-hertz), puts excess strain on the cpu. Thusly decreasing its life span. So increasing it, or overclocking as it is referred to, is really a delicate art. Trading off durability and reliability for performance to better meet your needs. When done right, its a good thing. When done wrong, its a very bad thing.
EDIT: I just realized you were talking about your pci-e slot. So everything I said holds true, it just holds true for whatever is plugged into that pci-e slot instead of the cpu.