That actually is hardcoded into the dosearch.php script on about line 95. There is a line that should look like this:
if (!$Limit) { $Limit = 25;}
Yours probably has a 50 there. You can change it to a lower number.
With your server specs you're definitely going to want to bump that down quite a bit. For any mysql application that gets a fair amount of use, memory is one of the key components. mysql LOVES memory, the more the better. Most users will at least have a GIG of memory for the server.
What will happen is your forum will gradually slow down the more users/posts/messages are stored in the database. If upgrading the server, throwing more memory at it, etc. is not an option then you'll want to look at activiely pruning topics and private messages to keep the total amount stored in the database down to a reasonable amount.
