DRM theoretically makes it impossible to copy DRM-protected material. It has no effect on, say, TV shows you download through bittorrent or mp3s or anything else like that since that stuff isn't DRM-protected. It's basically only going to be relevant to media that you have to purchase to download.
None of your files will get deleted. I don't know how this rumor got started, but it's utter nonsense.
With those specs, I bet Vista will run faster and more smoothly than 2000/XP. The old OS doesn't make effective use of most of that memory. Vista and Aero are not going to bog down your video card - a GeForce 6200 is adequate for Aero effects - although the drivers don't yet deliver 100% of the gaming performance you got under 2000/XP.