Well I'm not sure if you guys have noticed this or not, but on the PS3, even if you have a 1080p enabled TV and also have the 1080p option enabled in the display settings of your PS3, GTA IV will still run in 720p on your TV. For some reason the PS3 decides to forgo the 1080p option and output in 720p (I know this, as my TV states whether it's running in 720 or 1080p). Even if your TV, Game and Console all can run in 1080p.
So the way to get around that is to turn off 720p (and 1080i) in the Display options on the PS3 (probably the same for the X360) and basically force the game to run in 1080p. For me, it resulted in a MUCH better picture. Before, when I first bought my HDTV, GTA looked awful on it, it honestly looked better on my 15" SDTV in 576i, but now that I've basically disabled the option for the PS3 to put out 1080i or 720p, the game is now running in 1080p, and it looks much better.
Now before you guys tell me that the game is being rendered in a lower resolution, yeah I'm aware of that, I know that GTA IV on the PS3 isn't even being rendered in 720p, something like 620 or something. But I still noticed a big difference in forcing the PS3 and my TV to upscale it into 1080p.
This can also work in a few other games where you can "force" 1080p, mostly it says on the back whether it can or can't, and mostly it does it automatically but for some cases, like GTA, you have to force it to get the desired resolution. If you try and force a game that simply can't run in 1080p, then it'll run in default 576/480 SD mode, so you'll have to go back to the options and turn on 720p or 1080i
Well I'm just posting this because of the big increase in picture/graphics quality, seriously this game looked like shit when I first played it on my HDTV. You know that shitty effect where it looks like the games graphics is watercolour painting in motion, well it doesn't look like that anymore
Also get a good sound setup, feels like I'm playing a different game and all I did was add a half decent DELL subwoofer from 2001 to my setup