I like Marc Gasol a great deal but I think Memphis might be best off trading him. He'll have terrific trade value and will be able to bring back a talented player.
Hasheem Thabeet, on the other hand, I'm not convinced teams will give up a major asset for him. Not after the 10mpg he's received this season. I think they'd be better off keeping him and developing him. He'll offer more value down the road than what I reckon they could get in a trade.
Considering Wallace's reputation around the league as a sucker (deservedly earned or not), I imagine Memphis would get very little for Thabeet right now.
The biggest problem with trading Gasol is the Grizzlies lack of contracts above $5 million. Only Zach Randolph. That is a sizable complication. Could make the Grizzlies life difficult when matching salaries. Limit them to rookie scale players. Or force them to make a larger trade.
Still, I have to believe there would be a lot of teams very interested in a player who's putting up a very efficient 15ppg + 9.5rpg. Also, a good passer and a combined 2.7 blocks + steals per game.
Memphis should be able to find a good trade despite that complication.