ONLY 70000 FACES? 100MB? Yeah, you do know that whwen you do anything, with anything, it creates a node right? Like when you select the item its like PCube12-P12Cube12Shape-PCube12MatteOptions-PCube12Tweak32-PCube12ExtrudeFace... etc. When you delete the history for everything... all of those nodes DISSAPEAR. It helps alot because those nodes are saved, and in intense modeling they are created excessively. Well, 70000 faces isnt that much... shouldnt take up that much space. I have a file with 200000faces that takes up only 3.5 MB. So, it has to be the history problem. Thing is, I have an Athlon too, but mine is a bit stronger then yours, so 70000faces might be much...

Cheers,
,Matt