With that computer, you won't be able to use all the cool Aero features. If you have upgraded it a lot, you may be able to use to use it. You'll have quite a difficult time running Windows 7 with this computer. Minimum requirements are there for a reason.
EDIT : My buddy tried to install on a AMD 1800+ with 2GB of RAM, and a FX5500. While it did install, it doesn't run very great.