I'm with you guys on all of that. Just like dano, I will play the crap out of something, then lose it for a bit, get totally caught up in something completely different, then flick back and repeat. And I rarely trade games in. It makes me feel like I'm losing out. I especially hate trading in games with big multiplayer modes, because they never really feel like you're "finished". I guess that's the point.
But I have only ever re-downloaded one game - Ninja Gaiden Black. I won it in a competition but didn't have space on the 20Gb drive for it, so it had to go. I forgot all about it and after I'd imported my profile to the new HDD I downloaded it again. Most of my space was taken up by DLC for games, rather than anything else. Tracks, cars, setups and skins for Forza 2 (about 2Gb), the masses of patches for Burnout Paradise (about 4Gb), modes and stuff for Gears and Gears 2 (2Gb or so), tons of extras for Mass Effect (I haven't bothered with the premium stuff for ME2). It all adds up.
I fully expect I'll pick up GT5 when it comes out, and it'll be great since I can use my new wheel with it. It had just better live up to expectations. The online functionailty and community will make or break it for me. There's never that many people in FM3 online, and the ones who are tend to be a little... cold. Yes, I suck. But that doesn't mean they have to totally blank me.
And I can tell you right now that the most used track globally will be the TG Test Track