I'm not sure what TK is (I'm an eternal newbie here as you see) but you can savely install i386 into your machine, it won't do any harm. It's said that i686 programs are, indeedly, optimized for P4 Processors. I've a P4 myself too and, I see praticaly no difference between those packages (i386 or i586 or whatever)....
You probably would like to give gaim a try too

It's a good instant messanger if you need more then msn together in one program (like ICQ or AOL).