Hi jdoolen - Welcome to w7forums
If you would please run
Windows 7 Upgrade Advisor.
The advisor will tell you if your system will support Windows 7.
How much Hard Drive space are you talking about? Windows 7 will need about 15GB to install. Can you move some of your data to the external drive while doing the install?
As a safe note, It is always advisable to backup your data before installing a new OS.
On a second note, It is always best to do a custom (also known as clean) install instead of an upgrade. A custom install can be done to different ways. You can install and leave all your data in place. Windows files will be copied to a Windows.old folder. Or you can format the drive before installing to ensure there is nothing from the previous install to damage the new install. I and so many others prefer formating to ensure even less problems. However doing a custom install of either choice will not carry over your programs. All your programs will need to be re-installed.