For surfing the web and downloading files, it is doubtful that you would see any issues with one etherdongle. 1Mb per pixelnet universe + overhead is still less than 5Mb per second. If you've got pretty good wifi reception, your link should be able to carry many times that.
Where you are likely to have issues is if you are copying files between machines on your home network or streaming media from a home server.
Most isp connections are not going to saturate a home network.
If you have any issues, it is most likely going to be from poor wifi performance.
RM