Thanks for the feedback guys.
Yes it is an i7920 CPU and no I don't have view distnace at 10,000 - more like 5,000 since trying to drop down all of my graphical settings to avoid frequent crashing to destop (which still hasn't helped).
I have only been playing the single player campaign so far but will give MP a go to see if it runs any better. Fingers crossed...
I do intend to upgrade to Windows 7 sooner rather than later, but I really balk at the idea of having to upgrade my OS just to make a game run properly... even if it is a damn good game.
The reason I went with Vista 64-bit was to get access to the extra RAM that I assumed would help most games (and for database development which loves the RAM) but it sounds like perhaps I was misguided...
Anyway thanks again for the feedback.
Will soldier on for a bit longer with it before throwing in the towel completely.