20gb xbox is worth peanuts these days. I work in HMV, and we hardly ever get them in, price is really low. For the games I'd say 30 each, average, for the 3 recent games, (battlefield 3, mw3, and skyrim) and... The price would vary on the others. The halo games aren't worth much if I recall, nor will guitar hero, might get a tenner for battlefield 2, the call of duty games still fetch a reasonable price.
You'd be better going on adverts will all that, just know the original xbox 360 models don't sell for much these days./