How do I determine the capabilities of my wireless network adapter?

The quickest way, of course, is to look at the paperwork that came with your
Dell. They're very good about describing the capabilities of things like
network cards as they enumerate the features installed on your machine.

But, of course, that's not an option for everyone. Fortunately there's a
relatively quick way to check for what you need.

Head to Control Panel, and open up Network
Connections:

Right click on your Wireless connection, and click on
Properties:

Next to the "Connect using:" box, click on the Configure...
button:

Now, this dialog may look a little different depending on your specific
network card. However almost all will have an entry under the "Property:"
setting that's labeled "Data Rate", "Speed" or something similar that will
expose the speed capabilities of your network card and driver. Click on
that.

In my case, I also have to uncheck a "Use Default Value" checkbox
to enable a companion dropdown box listing available speeds.:

As you can see, my card is capable of 54mbs.

There's no need to change anything. The default or current
setting should typically be correct. Just hit the ESC key, or click on
Cancel to close everything back up.

The important point here is that this approach allows you investigate the
capabilities of your network card, wireless or otherwise. If you don't see the
54mbs listed, or 801.11g isn't an option, then your card may simply not support
the higher speed connection.

Now... is it worth it to make sure that everything's at 54mbs?

That depends.

First, be aware that you may not experience the full nearly 5x speed
increase if you do switch from the 11mbs 802.11b to 54mbs 802.11g standard. My
own experience is that the actual speed is sensitive to both distance and other
electrical interference. I got about a 3x speed improvement, even though all
the equipment is connecting at 54mbs.

It also depends on what you're doing. If you're copying large files between
machines, you may notice the difference. If, however, you're simply surfing the
internet there won't be much change since it's more likely that your internet
connection, and not your wireless, is the slower choke point. It may not make
that much difference if your wireless is 11mbs or 54mbs if the internet
connection is limited to, say, 768k DSL download speeds.

There's certainly no harm in having a mixed environment. It's quite ok to
have some wireless operating at 11mbs with equipment that's capable of handling
more. In most cases, you'll simply be limited to the slower speed. And as I
said, whether or not you'll even notice, depends entirely on what you're
doing.