Sell them. Actually, I just gave my one of my roommates my entire Xbox collection (which is about 8 games - mostly all less than a year old - because I generally sell them all back). Thus begins the retirement phase. I'm now a Wii only gamer.
Seriously, if you're probably not going to play them again, why keep them? It doesn't return any of your cash and it just adds more clutter. And I hate clutter.