Well, the next GTA will be portable on PSP, we all know that. And its a no brainer GTA4 will be on PS3. And probably will be released for the XBox 360 (damn that long name!) and PC after a year or so for sure.
As for the difference, yes, since its a GTA4 and not part of the GTA3 family (GTA3, VC and SA). I think there's been an interview with R*'s Sam Houser that pointed out that the next GTA will be totally different in the mission structure, narrative and graphics-wise.
Question: Is this the last new Grand Theft Auto game we'll see on this generation of hardware? It seems like you've probably pushed the PS2 hardware about as far as you can.
Answer: "Terry [Donovan, CEO of Rockstar] will batter me if I answer that question! [Laughs] It is the last full-blown thing we're going to see for now. But someone might come up with a wacky idea that's fun and relevant tomorrow. I'm one of those "never say never" kind of people. It was really important for us to do this trilogy. A fictional New York in a contemporary setting, Miami in the '80s, and a '90s Cali - that was the trilogy. That was the plan from a long time ago. I'm not the biggest fan of games coming out with new versions year after year. Putting out the same game every year, plus ten percent, that's not our thing."The domans
http://www.grandtheftauto4.com/ and
http://www.gta4.com/ have been registered.
Go to the
GTAForums.com, people are already discussing this.
Some people are saying we will be seeing another "series" like for the GTA3 trilogy. That would make good use of the engine and the console's lifespan. It will probably be online playable and I think we will be downloading new skins etc just like on PC. That would be nice.
Goto:

Release Date: October 2006? I doubt that. I'm seing a late 2007 release, too much work this time around with the team having to get to grasp with the new ware first. And GTA3 was released two years and a half afte the launch of the PS2.