However, if your WiFi box is all that's between you and the Internet,
your maximum speed will be that of the Internet connection even if
you have a Gigabit LAN setup.

ADSL speeds are typically no greater than 6Mbps and often 1.5Mbps or
even less.

Cable speeds are typically 1.5Mbps to 6 or 8 Mbps and can go as high
as 20 Mbps (or higher) if you have a DOCSYS 2.0 modem and your cable
service can provide that level of service.

Verizon has an FiOS service (fiber to the home), but they're not in
my area and I don't know what speeds they provide.

If you have one of the above 3 Internet connections, you will not
see any speed improvement to/from the Internet by using a wired
Ethernet connection.

If you have more than one system and you're copying stuff between
those systems via your WiFi LAN, you will see an improvement by going
to a wired Ethernet, but that improvement will be achieved only
between those systems and not to/from the Internet.

If you have two or more systems and you want them all wired together,
please read on.

There are basically two ways to setup system(s) on your local LAN:

- static IP (e.g., 192.168.1.*, 172.16.*.*, 10.*.*.*, etc.), and

- DHCP (where your system receives its IP and DNS info from a server
which is commonly your DSL/Cable router/firewall, or even your WiFi
box)

I'm guessing you are already setup for DHCP since that's typically
what a WiFi box will provide by default. I'm guessing also that your's
has two or four RJ-45 LAN Ethernet ports on it for direct connection.

A wired network configuration on a system is twofold:

(a) device driver for the Ethernet interface, and

(b) software setup for the connection type you want (static or DHCP).

This reply is going to be a starting point because you'll need to
provide some more info.

The first thing to do is setup (a). With your Ethernet cable attached
to both your system and the WiFi, do this at a command line:

/sbin/ifconfig -a

The GUI way is two-fold:

From the (Ubuntu) top menu bar:

System -> Administration -> Hardware Drivers

mine doesn't display anything, so that means whatever Ethernet driver
is on my system came with Ubuntu. Then:

System -> Administration -> Network Tools

That should indicate if the Ethernet interface is functioning.

To proceed, we need to know what information was displayed. Also the
present setup you have; for example:

[Cable modem]====[Router]====[WiFi]

and how many (and what OS) systems you want to connect.

If you want to simply jump ahead, here are some good references:

static IP: <http://codesnippets.joyent.com/posts/show/319>

DHCP and switching back and forth, some info here:

<davestechsupport.com/.../how-to-setup-internet-connections-i\
n-ubuntu/>

The above URL looks really good. Other info here:

<ubuntuforums.org/showthread.php?t=447361>
<ubuntuforums.org/.../t-489491.html>
<https://bugs.launchpad.net/ubuntu/+bug/146990>
<https://lists.ubuntu.com/archives/xubuntu-users/2008-January/001019.html>