If it's just for Vista and your programs (i.e. games, documents etc on other partitions/drives), then about 20-30GB, I'd say. I've got mine at 20GB and that's fine - games are on a different partition, docs, music etc are on a different drive, as is the pagefile. Only thing chewing up space (3.2GB) is the Hibernate file, which you can safely delete in most cases using Disk Cleanup. Only reason I'm using it is because sleep doesn't work properly on my PC at the moment.
Even with all that, I'm sat with just over 4GB free, which is safe.