Halo 2 is a great game, but its real gem is its online multiplayer....
With that said the Graphics have greatly improved since the last game and some of the character designs have changed aswell. But its mostly the same stuff.
The sound in the game is not the best I've seen games like God of War have good sound but the sound isnt really there like this game. Its decent you can hear the aliens down the hall and you can here the soldiers screaming but thats it, nothing special.
Finally, what you've all been waiting for the online gaming. Halo and Halo 2 have some of the best online communities you will ever see in a online videogame. The online provides games such as Capture the Flag(like the old one), King of the Hill etc. Mostly the same things. Same as single player you are able to hi-jack enemy crafts by kicking them off or punching them, its a very nice feature.
So there you have it Halo 2. Halo 2 is a great game, if you like Sci- Fi shooters I recommend buying this game, if not you should check it out. Its also a good addition to Xbox live so I still recommend buy it.