I'd say the two critical things that make the biggest difference (to me) are the brakes and the fork. If your brakes are OK but you're unhappy with your fork then definately change the fork when possible. If you have v-brakes then it's better to upgrade them to discs and THEN change the fork (if you feel the need to).
One thing: am I right in saying that the F600 has either a Headshok or a Lefty fork? If so, I don't think you'll be able to change it (could someone please correct me if I'm wrong?) I'm not sure about this though.

Ride all the bikes and buy whichever feels best to you. =)