While the system requirements are not yet set in stone, though I think they're pretty close to the mark; we will not be artificially inflating them. Despite what some may think the vast, vast, vast majority of PCs are not state-of-the-art, they are at least 3-4 years old (believe it or not, Schem is right on this point
). This goes back to the limiting of potential market stuff I tried to explain in the 3D thread. 
Higher system requirements does not automatically mean more features in the game either. That stuff depends on development time and, more importantly, budget. All these features cost money to put into the game. For example, with GalCiv II there were people who wanted every race to have custom building artwork on the colony management screen. However, adding that in would have meant cutting out custom ship building entirely - which is a much better feature that everyone loved. Given the choice, I think most people would rather have something that's fun. 
Well here is the thing. I don't think we're talking about a High-End exclusive game, and up until this point I don't think any game is really.
I have 2 gaming machines.
+ A 4 years old machine: AMD 2500+ 1GB RAM Readon 9600Pro.
+ A new machine I built a few months ago: Core 2 Duo 6300 2GB RAM Geforce 7600GT.
And up until 3 months ago (when I acquired the new machine) I played all the high end game on that old machine of mine and they all ran perfectly fine and I was happy even I can not get the most out of those games, they're perfectly playable. Want some example? Oblivion, I played it for about years, beat the main story, explored every corners ...etc... I got a few FPS drop here and there, and certainly the detail was on mid low. But was I able to play and enjoy the game? Yes. Or X3: The Reunion. Beat the story line, built myself a huge big trading empire just like a person with a high end machine. The cons? The old machine can not handle the level of engagement if I jump into a Xenon or Khaak sectors.
Now, with this new machine. The first time I played Oblivion on it, I said:
damn this looks good boys!!. And the FPS hiccup also disappeared. And now, playing X3 I can finally engage Xenon sector by myself instead of doing OOS like on the old machine.
So, the thing here is that we're talking about how a game can be played on a wide range of system. The people who has limited resource can still play the game, but also, the game should be able to ultulize the extra resource that may be given to it to enhance itself. I didn't say, neither prefer a game to be high-end exclusive, like you said, it's bad for sale. But neither I find agreement the agressive low end defense (i.e from Scheme) that a game should be made cartering for low end machine and it should not demand any more then that. Low end can play the game, high end can play the game as well, but it's quite reasonable that a high end machine should be able to provide a better experience. On the other hand, demanding a game should be play at the same level on a low end and a high machine, IMO, is much less reasonable.
Plus, it's not like Sin is released today or tomorrow, it's scheduled well into next year isn't it.