Try this! This is a new Greasemonkey script that can change your experience on Gmail. I always thought the normal conversation preview is not too useful as it shows one sentence only.
Over Persistent.info has a Greasemonkey script that able to add a preview bubble which shows the first message and last message for preview. It is extremely useful:
… One of the things touted by the upcoming Yahoo Mail and Hotmail releases is that they will have a preview/reading pane which will let you see message contents at a glance without having to navigate to an entirely new view. Gmail offers a lightweight version of this already, by showing the first hundred or so characters of each message as a snippet next to the subject. While this is handy for one-liner emails, a full-blown preview pane is often more appropriate.
Given my past experiences with Gmail and Greasemonkey, I figured that adding a preview area to Gmail may just be possible. Ignoring the technical aspects, the first issue was deciding what it should look like. My main issue with traditional preview panes is that they take up a lot of room, even when they aren’t needed. Eventually, I was inspired by Google Map’s bubbles and decided to try that approach…
Make sure you have Greasemonkey 0.5 installed. It is needed to get this work.
Gmail Conversation Preview Bubbles – [Persistent.info]