I recently decided try Windows 7 x64 RTM on my primary media center. While my experience was much better than when I did the same with Vista as it relates to drivers and codecs, the overall experience wasn't great. Granted, my hardware's not as beefy as yours - I have a dual-core Athlon and 2GB of RAM. I found that the overall responsiveness of the computer (dedicated to media center) in both native and extender modes to be sluggish. I went back to x86 yesterday, and my media center is back to being snappy again. Based on that experience, I'd recommend x86 for media center.
For the record, I am running Windows 7 x64 on my laptop which has 4GB of RAM, and I'm seeing about the same performance as I did x86 on the same hardware.
That's not definitive I know, but gives you another real-world data point.