The thing to remember is that the cap will likely increase at a lower rate than the payroll for existing contracts will. And yes, any players signed this summer would count against the cap as well.
I'd be inclined to oppose waiting if a great player is available. If not then adding someone decent for a reasonable contract to replace DRob in the lineup wouldn't be a bad idea. If the Spurs are that concerned about Ginobili then they could preserve enough flexibility to offer him a decent contract in 04.