It depends on if both computers are utilizing the network. For example, if there are 2 PCs connected to a 54MB router, and only 1 PC was actually using the network, it would get 100% of the network bandwidth. If both were using the network, then the router would start dividing up the bandwidth. Note that 54 MB is a very large amount of bandwidth and unless you are working at transfering very large files you will never see the difference.
The effect on your actual internet speed may be easier to see. Example, my cable ony gives me about 4.5 mb downloads. That bandwidth is essentially being shared between the 2 PCs as well. For normal web browsing you should not see any effect as that usually requires a small amount of bandwidth. If you start downloading large files, or looking at a bunch of YouTube, or have 20 torrents open at once, then you could start to see one PC taking more bandwidth than the other and slowing down general browsing (for both PCs actually).
Note that it is the actual internet connection that is being saturated during this as the 54MB router has more than enough bandwidth available to deliver the 4.5MB internet connection.