Depends. With a target man, i always seem to get great success with someone who can header. IE Toni @ Fiorentina in FM2007 (Although hes moved to Bayen

)
Having him and Mutu/Pazzini. He could flick balls on to them, spliting the defence.
Of course theres times when he can play bad, and it will really affect the team so you will have to have a change of tactics at hand. Esp if you come accross a worldclass CB on a good day.
Lower leagues i always try and sign someone with good heading skills so i can play quick counter attack hoof the ball up field to him and hope his header goes towards one of my fast wingers or my other ST!
Playmaker
I've never really played with one myself, although if i was at Arsenal i would as Cesc is great. He might get 1 cracking pass that gets you a goal and wins the match. But then, if you have someone like Hargreaves, Gerrard man marking him. You team wont function properly and again, you will have to think of something new.