[finas]

Using a DX1480 and a regular xbox with 128mb, I can confirm that:

in both cases, setting the file to "no cache" improves playback using dvdplayer or mplayer.

I am using 4 files as test samples: the trailer from ocean's 13 downloaded from apple site in all it's versions: regular, 480p, 720p and 1080i.

with cache on, 480p drops frames on both 733mhz xbox and 1510mhz xbox, but there is a slight decrease in dropped frames on the 1510.
with cache off, 480p drops alot less frames on the 733mhz ( it only drops a few ) and on the 1510mhz it plays perfect.

with the 720p file, with cache on, the file is unplayable on the 733mhz xbox and on the 1510. Setting the cache to off, improves the playback but not to the point where you can actually watch it. The difference between the 733mhz and the 1510mhz xbox is small.