I found a better and easier way to do this!
First of all, I don't have much php or mySQL know-how, but I guessed, and managed it myself, and it seems to work.
I went to the forum.php file in /admincp and copied all lines of code pertaining to the description box, then renamed the new instances of "description" to "welcome". This gave me a new box and variable in the edit forum page, in the ACP. But when I went to edit that box, I got a mySQL error, so I went to the database, and looked around till I managed to create a new row in the forum table titled welcome.
Then I went to forumdisplay.php, and copied the code regarding the forum 'description', and renamed to 'welcome'.
Then I placed $forum[welcome] in a table in the forumdisplay template, and voila! The text I had written in the ACP now shows on the forumdisplay! Now I don't need 30 lines of < IF > conditionals, and the welcome messages can be changed easily in the forum edit.
I suppose someone could make this into a simple mod, if they knew how to make the database edit from a .php file.