A relatively quick survey.

[eluser]Dam1an[/eluser]
You're creating a forum? How did I miss that?

One option would be to keep it normalized and do the count in real time, but thats not what you're asking, I I'll assume you've decided you're definatly storing the post/thread count

From the 'make it as easy for the user' point of view, I would go with a seperate table, which would have user_id (both PK and FK), thread_count and post_count.
Maing users manipulate their current tables will potentially put off a lot of users (also, as unlikely as it is, what if they already have those columns in the user table, or whatever names you choose)