The 20gb is definitely enough for regular gaming and stuff. I never really even ran out of space on my old Core 360 w/20gb hd.
I ended up later getting an Elite, because I wanted the black system, the extra space is always nice to have(though I have +90gb free, haha), and my old system didn't have the hdmi plug(all of the newer systems do). The extra hd space is good if you want to dl a lot of media, or xbox classic games, or put your own files on the hd. There are no game installs on any of my 20 Xbox 360 games, but I'm not sure if other games might have them.(most likely not)
starcraft: "I and every PS3 fanboy alive are waiting for Versus more than FFXIII.
Me since the games were revealed, the fanboys since E3." 
Skeeuk: "playstation 3 is the ultimate in gaming acceleration"
