Darko is a restricted FA, so the Magic will have the right to match any offer made. The C's could offer him a full MLE with a 15% trade kicker (ala Blount), but the Magic would match it, and Darko wouldn't sign for that kind of cash (6 years and $40 million or so). Darko's camp want Tyson Chandler/Semual Delambert money, meaning $10 million+ over 5-6 years. I'm not sure anyone is going to give him that kind of money, but in this league, you never know. That being said, the C's could do a couple of things, as I too am intrigued by Darko.
#1 They could give in to his demands and pay him $10+ million a year. They would have to do this via a sign and trade since the Celtics are over the cap. Orlando would therefore have to take back an equivalent amount of salary in return. This would be possible because the C's have Theo's $11.6 million salary which expires after the coming season. However, Orlando isn't going to do the C's, an eastern conference rival, any favors by taking on $12 million for nothing. So, the C's would have to give something to Orlando to make it worth their while. Think Theo's contract and a 1st round pick, or Theo with a player like Tony Allen or Ryan Gomes. So, you'd be sending out Theo + either Gomes, or Tony Allen, or a future 1st round pick, and receiving Darko, with his 4 or 5 year deal at $40-50 million bucks.
YIKES!
#2 They could offer him the full MLE (5 years about $38-40 million) and hope that the Magic don't match it. This would have to happen ASAP, since Orlando would lose the first year total of the deal from it's current cap space. The Magic are +/- $12 million under the cap, and would be $12 million - $5.6 million, or $6-7 million under the cap if they matched. This would severely hinder their ability to sign a major free agent like Rashard Lewis. Of course, Darko wouldn't sign for the full MLE until he sees that no one is going to give him $10 million per.