Your links won't all work properly, i.e. the /user/username one won't work to an ID number, you could pass the username through then do a lookup based on the username, but htaccess won't know that username "tim" is id number "123"
This should work:
RewriteEngine On
RewriteRule "user/([0-9]*)" account-details.php?id=$1
RewriteRule "category/([a-zA-Z0-9_-]*)" torrents.php?parent_cat=$1
RewriteRule "browse" torrents.php
RewriteRule "" torrents.php
With that I've separated out the categories into a subfolder called 'category' this makes it far easier to allow any category name you want.
For the regular expression matches in the RewriteRules theres an excellent free program called 'Expresso' that will help you build up regular expressions and test them against a list of values to see if they match, I'd recommend searching for it and getting a copy as it's very handy.
Also for the username part if you wanted to do username instead of user ID numbers, then swap:
RewriteRule "user/([0-9]*)" account-details.php?id=$1
to this:
RewriteRule "user/([a-zA-Z0-9_-]*)" account-details.php?username=$1