In all honesty, I have no idea. I only have three machines to base my suggestions on - mine, which is basically the same as yours except for an X1800XT 256mb graphics card (and an E6400 - no overclocking) - and two machines which run high easily, but are both very much more up-to-date than mine.
If you asked me to make a wild guess, though, I would say that a decent stable overclock and an 8800 GTS should be able to utilise High Detail.
Bear in mind that, even on Medium, I think the game is very pretty (and even not counting the graphics it's just bloody good fun!).