Macfrog,

One gig of RAM is ridiculous. Even for a Mac.

If you can open your file, you may try these things:
- delete all plants and trees
- if you have used your own, imported textures, check that none of them is bigger than 1024x1024 pixels

You may want to share your file so we can have a look at it. Try using www.mediafire.com to do so.

Hans