No, it won't make cable internet go any faster. Using real world maxes 10Mb is almost twice as fast as the fastest cable I have ever seen (about 4Mb) and 100Mb usually tops out at about 50Mb. Gigabit is theoretically capable of 1000Mb but in reality you see more like 300Mb. This was on a small test LAN of just 2 computers and a gigabit capable hub.
So the short answer to your question is that your cable connection is the bottleneck so faster LAN wont increase its speed. It would however be faster for machine to machine transfers in your home provided you also get a gigabit capable hub. But it is backwards compatible so a lot of people go with the gigabit just so they have it. If an option is ony a couple bucks more it can save you lots later.
Anasazi your math would work for a serial connection, ie. one data line. But the PCI bus is a 32 bit bus operating at 33MHz(generally, there is 64bit and 66MHz out there but not on most desktop motherboards). The max data rate of PCI is listed at 264MB/sec on the 64bit version and 132MB/sec on the 32bit version. These are of course theoretical maximums.