Before you go paying off debts, you'll want to know the SOL and DOFD. If it's a debt that went delinquent in 1998, then it can no longer be reported on your CRs and in most states it's beyond the SOL so they can't sue.
If the debt is recent, then you should try to negotiate a PFD. Paying off a collection or chargeoff will simply yield a paid collection or paid chargeoff. It won't help your score.
Prime lenders generally require unpaid collections and chargeoffs to be paid, but that doesn't mean you shouldn't try for a PFD. Worst case scenario, they say no, and you pay it. Be persistent and start early. DW and I are planning to buy a house soon, and I'm trying to leverage a PFD outta the one paying on collection we have.