iPhone 3... How much available memory should I leave? How low should I let it get befor I stop downloading all kinds of apps and stuff?
You can use it all if you wish. It shouldn't noticeably effect performance unless you have to many apps running in the background. I have filled mine up and had it almost empty, it makes no difference.
I have a 3GS and I filled it all up and it ran noticeably slower but other than that it doesn't really matter I wouldn't think.