On new windows computers with enough RAM and no other reasons for a memory shortage, you may get a message:
"Cannot save plan file, not enough room in heap zone."
Rather than spending hours trying to resolve and losing information, because of inability to save, see if you have Nero software for your CD/DVD drive which is trying to index your ArchiCAD PLN as you work.
Disable Nero Indexing. You'll see it in Task manager as:
NMIndexStorageSvr.exe
This uses more memory than ArchiCAD itself, and prevents even medium sized projects from being saved even with no other applications or windows open.
Here's information for removing the Nero search:
http://www.neowin.net/forum/index.php?showtopic=488539&st=0&p=587807940&#entry587807940