Go and look at Yanfeng's posting:
He lists a pretty good machine. You can update the graphics cards or make some of the other suggestions for changes.
If you decide to load Windows 7 Pro 64-bit you will have to run 2007.8 in the Widows XP virtual machine environment. You will take a hit on speed for doing so. But 2007.9 will run natively under Windows 7 in a matter of weeks from now.
Use at least two monitors. The Expedition/DxDesigner software is not written with two monitors in mind, but the small pains that come from having some dialog boxes needing to be displayed on the master monitor are easier to live with than not having enough screen space in which to work.
If you decide to load Windows 7 Pro 64-bit you will have to run 2007.8 in the Widows XP virtual machine environment.
I've been using this configuration for about half a year and it runs pretty well and without any noticeable performance degradation. But I hope there will be a native x64 port for EE2007.9