Let's try and take one question at a time.
First, the netbook.
You'll be well advised to install the dedicated netbook edition - click here
This comes with the rather futuristic Unity user interface - similar to Android on mobiles.
It is normally installed from a USB stick.
You simply follow the instructions provided by Ubuntu to make a bootable USB stick and install from there. It should dual boot without issues, providing you have enough hard drive space for both systems. Alternatively, run it from a USB stick directly, albeit at a lower speed. click here
Second, your old Gateway laptop.
If it's running Win98, it's maybe marginal for Ubuntu, but it depends entirely on what the specification of the machine is.
click here and see if it meets the requirements.
If it doesn't, don't worry, there are a number of Linux distributions (distros) designed to work on low powered machines.
A really good one I've found is Puppy Linux, click here which will almost certainly run on your laptop.
If I can help further, please ask.