iChat 7.0, how can I see previous conversation
For chat transcripts created by Messages, once you've selected the conversation in the Messages window (with File > Recent Items if necessary), you just scroll back and Messages will load older transcripts incrementally.  However, this is pretty poorly implemented, as there's no way to scroll all the way back at once; you have to scroll to the top or press Home repeatedly.

The newer transcripts are stored in ~/Library/Messages/Archive, not ~/Documents/iChat.  Quick Look is also supported for chat transcripts.

Chatology is a third-party app that may be of help; it will read transcripts in both locations, is faster and less buggy than Messages (e.g., lets you see images in old chats where Messages won't).