When to cache a Django based forum

I have a forum (http://forum.hudsonbelfast.com) written in Django/Python that essentially consists of the following model setup:

• Post
• Reply
• User
• Feedback ('starring' posts)
• Private Messages

Reply is specifically structured like this:

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class PostReply(models.Model):
    user = models.ForeignKey(User, related_name='replyuser')
    post = models.ForeignKey(Post, related_name='replypost')
    created = models.DateTimeField(auto_now_add=True)
    content = models.TextField()
    ip_address = models.CharField(max_length=255)

The views are fairly simple, viewing posts, viewing profiles, registration/authentication flows, etc. However it is beginning to hit a few limits with regards to memory. I've investigated a bit with mytop, and nothing really stands out as being terribly inefficient. The only thing I have noticed is that the Reply objects are close to 100,000 now, but I would imagine a DB shouldn't have a huge problem with this. So a few questions:

• Is there a way to have mytop log things so that I can see 24 hours worth of data and investigate exactly the queries that are causing problems?
• Does anyone have any suggestions for good ways to cache this setup?

To clarify, I understand how to cache when it comes to Django, however I'm finding it tricky to imagine a good way to cache my views and maintain a real-time feel to the forum (which tends to have quite a fast turnover in new posts). Caching views involving posts or replies seems like it would inhibit that real-time action. Ultimately I could start archiving old posts, however this would be a last resort.