The bare minimum hardware requirements for Windows Vista are 512MB of RAM, a 20GB hard drive with 15GB of free space, and a processor faster than 800MHz.
Depending on how much RAM you have, it looks like your PC will meet these requirements. However, a major caveat is that these requirements will give only the "core experiences" of Vista. To take advantage of all the cool stuff and make shelling out for the upgrade worthwhile, you'd need at least 1GB of RAM, 128MB of graphics memory, a 40GB hard drive and a few other extras.
More info on the requirements for Vista is available on Microsoft's Web site.
We also have a feature on Vista's pros and cons that should help you decide whether upgrading is justifiable.