The reason they say that is because Vista is a resource hog in comparison to XP. It's not a literal requirement, it's more like a guideline. Crysis won't take up 3.2ghz of your CPU in vista.
The dual I linked you to is 2.7ghz, but like with the GPU, ghz is not the only measure of speed.
FYI, Win7 is a lot more resource friendly than Vista. here's the download link:
Windows 7 Release Candidate Customer Preview Program
And Harnar, first off, the 4770 is 110 bucks on newegg right now. They are no long worth getting, since the 4850's are under 100 bucks in some cases.
And the GTS 250 is better than either of them, at a steal of a price. Hence, why I picked it.
EDIT2: Correction, 4770's are ranging from 100 to 120 bucks, and the only ones in stock are 120 bucks. No.