108.11 (a, b, g) are all just the name of the "standard" used (tells the techies the speed and range, but doesn't mean much to real people
) but the 54 part is the top speed that it's supposed to achieve (in MBits).
The Linksys WRT54G is a 108.11g 54MBit router so will work fine, as the incoming cable modem speed is only 10MBit.
Hope that helps.