I'm having the same problem. TV shows downloaded from the iTunes store stutter, and the sound and video get out of sync (video lagging if i remember correctly). Also HULU and Netflix play bad, same problem. I never had these problems when I was still running XP on this machine and also, AND I THINK THIS IS CRUCIAL, when i had Windows 7 32 bit Professional installed. I'm going to install Windows 7 32 bit tomorrow on a third, now empty, drive I've got in my machine and i will post whether the problem is gone.
BTW: i've got an ASUS P5Ne-SLI motherboard for which ASUS does not provide Windows 7 drivers, probably because they deem it too old to bother (lame). I've instead downloaded and run the 15.51_nforce_win7_64bit_whql.exe update directly from NVIDIA, the ASUS motherboard is based on an NVIDIA chipset. This did seem to help a bit, video seems to run better. But, alas, not perfectly like it used to when i was still running Windows XP 32 bit Professional. I just played Episode 1 of Season 7 of Curb Your Enthusiams in iTunes and the sound and video got hopelessly desynchronized after about 5 minutes.
The Windows 7 Upgrade Advisor give a thumbs up for all hardware it detects in my computer. It doesn;t make mention of my ASUS motherboard at all, so I'm not sure whether it is really compatible or not.
Next day update: I haven't installed Windows 7 32 bit again. I did notice in TaskManager that my computer was suffering from an overload due to the often reported Windows 7 Media Player Network Sharing Service. This Service kept my 2.66 GHz Core 2 Duo CPU at a base activity rate of around 32% with a lot of variation in the form of peaks and throughs. I disabled this service, and this helped play back significantly. There must be some sort of incompatibility between my hardware and Windows 7 64 bit that makes this service run really poorly. (This doesn't bode well for other applications that may hamper my system in a related fashion, not good.)
However, this is only a partial solution. Whenever the CPU gets taxed at 100 %, even if just for moment like for example when dragging a window or starting up an application, the sound and video in iTunes desynchronizes. It seems that for some reason the syncing sound and video is less robust than they used to be. Pausing the playback and restarting syncs the video again.
All in all, my experience with Windows 7 64 bit is very unsatisfactory. Sure, I can turn off services that seem to hog CPU, but there's no guarantee that the underlying problem that caused these services to use up so many CPU cycles is not present in other applications as well, thus losing a lot of the computational resources that I did have at my disposal under Windows XP. I will install Windows 7 32 bit again, suck up the loss of 1GB of memory (Win7 32b uses max 3GB RAM) and see if that runs better. If not I will go all the way back to Windows XP (that's still on a disk in my system, luckily). There was no real reason for me to move to Windows 7, it looking nicer was closest to what i had for a reason. All i do on my computer is in full screen mode: play video's, play games, connect with a server at work via Remote Desktop Connection, so I don't get to see much of the OS itself anyway.
UPDATE 2011-03-30 22:41: I solved my problem with out of sync video and sound (in iTunes and streaming from Hulu) on my Windows 7 64 bits PC and a ASUS P5Ne-SLI motherboard. I downloaded and installed the Windows 7 version Realtek_Audio_V51005904_V6015904 drivers that i downloaded from the ASUS website. Before I did that, Windows Resource Monitor showed that the System Interrupts gave a steady load of 4% CPU activity, whereas after the update that number has gone down to about 0.04%. Great! Audio-video plays well now on my computer. I find this a bit surprising because I had disabled the on board Realtek sound device on my computer (under computer>magage>devices) because I use an external TASCAM US-122L or a Logitech G930 headset as sound output devices. Anyway, I'm very happy things work, I hope this information will be of use to others.
