CAP1, the main reason for the drop in your frame rates is that your card, the GTX9800, has been told to be as the minimum for the new version. AH3 will likely be much more dependent on the video card than AH2, whereas the load on the processor will diminish. I have an E8500 (overclocked to 3.8) with a newer video card and my frame rate is fairly good even in the Alpha. Bus as has been said, your experience with your current setup is what counts the most. The alpha is a work in progress, so anything can still happen. Don't rush! Except for installing SP2 to your Vista.
As for the vid mem output, it apparently still doesn't show any correct numbers due to Microsoft.