I read a preview last week and the journalist had taken note of the pc spec of his demo session, it was a dual core C2D chip running at 2.5ghz with an 8800gt graphics card, everything was cranked up (including 8x AA) and he said the game never dropped a frame.
Sounds promising, I'd guess the game can be scaled down for older computers quite well.
On the subject of difficulty, games with interesting ideas and hardcore difficulty are actually being released, most of them are downloadable titles on the PSN or 360 Marketplace though, it's a start