In the current situation, biggest names are Lewis and Carter, as you said. But if we re-sign Gerald that would be the biggest free agent signing imho. And while we have Crash, paying big money to Lewis is just meaningless. And although we desperately Carter imho, there aren't suitable conditions to get him.
So going for other options(cheaper ones) is better. Next year, Okafor's extension will kick and we'll no longer have that huge cap space as much as we do at the moment. But signing players to splash would be simply betrayal to the club and fans.
What we should do is searching for trades with clubs which are trying to dump salary for luxury tax reasons.