TS is correct, the minimum you want mkv + AC3 (dts if possible).
In the container (.mkv) you want the video to be "AVC" (advanced video codec) and the audio to be one of those mentioned above.
They should have a high bitrate also, never less than 4500kbps.
You are getting them possibly in xvid or other standards not really accepted as good enough for HD.
If your not a quality freak then stick with them but if i were you i would get the scene rips by groups like -SiNNERS and -AMIABLE, -CBGB etc etc.
Beyond them get the top quality p2p internals from groups like -ctrlhd and -DON -HDChina (-HDC).
The p2p internals follow no ruleset so can have the best spec possible where as scene does the same but alters a couple of settings for best compatibility for players.
This tool gives you good info on your media files.
<a class="postlink" href="http://mediainfo.sourceforge.net/en/Download/Windows" onclick="window.open(this.href);return false;">http://mediainfo.sourceforge.net/en/Download/Windows</a>
Rule of thumb::
1:30m play time should be 4.4GB or so, 2H = 5.4GB for 720p.
A film like LOTR extended edition in 720p for example is about 10GB