This is likely the number one question asked about Revit. 'What's the best hardware configuration? '
As of today it seems that RAM is the biggest influence on performance. Most video cards that claim to be good for CAD work fine.
Chips are important, here's what to keep in mind; get the fastest good quality chip either by Intel or AMD. Revit rendering will take advantage of dual core. Revit is not 64bit ready but it wouldn't hurt to look at those chip offerings as well.
When you talk RAM you also have to talk Microsoft Windows. Simply stated, only 2 GB are available for any given operation. There are ways to get 3 gigs available to Revit but let's avoid the techno-babble.
Here's how, just in case: