Vista isn’t going to fix this problem, as the 360 is actually going to be doing the decoding. The media center just provides a clean interface to stream the files.
I’ve been doing it for about a year now w/ vista ultimate. My Desktop is on Wireless-N, while the 360 is hard wired. While i do sometimes have issues, it’s usually with music where it hangs for a little while, trying to list the 5k+ songs…
Also, over that setup (wireless…) i have been able to stream WMVHD movies no problem.