It’s currently unknown if GTA 5 will be released for PC, but it’s speculated that it will be released for PC a few months after the console release. It’s currently unknown how well the Xbox 360 can handle it, but it’s assumed that the PS3 will be able to play it at a very stable level.
The system requirements for GTA 5 shouldn’t be too much higher than for GTA 4, which is good since upgrading hardware is expensive. Also if it was much more intensive then the Xbox 360 probably wouldn’t be able to handle it.
Here’s a great site with information about this exact topic. They have a professional estimation of what the system requirements are likely to be.
How well it will actually play on the Xbox 360 will not be known until the game is officially released. It’s expected to actually play a little bit smoother than GTA 4 did, but whether or not that is true will be unknown until later this fall.
Check out the "suggested by the author" links below for more GTA 5 information.