<p>Similar to the one here on StackOverFlow, I would be needing to implement a way for people to vote up and vote down comments in a forum like site.</p>

<p>However instead of having a generic overall score, we will display the total amount of "thumbs up" and "thumbs down". The overall score will be needed for filtering purposes, such as "sort by highest rated", "show only ratings with + 3"</p>

<p>What is the best implementation strategy?</p>

<p>As a user suggested, I would also be storing the information who casted the vote</p>

Well you'll need to store ratings (Comment ID, UserID, Vote-Value) so you can calculate and stop duplicate voting but I would strongly suggest you also add a VotesUp and VotesDown fields on your main comment entity.
Why the duplication? Speed. You're going to be doing disgusting amounts of
```
SUM-WHERE
```
statements otherwise and they'll run your database server into the ground. A few extra bytes on the header record and you'll be able to sort and filter to your heart's content.
Edit: If you're going to also sort by overall score in some cases, you might want to add a third field (VoteTotal).
Edit 2: The duplication is pointless if you're able to cache all the comment headers in memory. That's a lot of data though and you'd need a ton of memory to effectively cache it. If you're not a billionaire, I'd just duplicate the data.