Although he spend all his career as a 4-3 End, the guy is 6-2, 245lbs. Even for a 3-4 OLB he would be slightly undersized (Ware is 265lbs). He is on the wrong side of the 30s though (he turns 31 in Ferbruary), so he would be no long term solution.
On the other side, he still has that pass-rush ability that Spencer never had. PFF gave him a +16.8 grade this season. Almost all of which came for pass rushing. He had 10 sacks, 8 hits and 30 pressures. His performance was a bit hit and miss (9 green games, 5 red games), but has been relatively constant these last years (19.2 in 2010, 32.2 in 2009, 12.3 in 2008). This season he was the 11th pass rushing rated 4-3 End. He is by no means a number one rusher, but as a complimentary guy to Ware, why not?
The Colts are unlikely to resign him and if the Cowboys could get him on a 3 year deal at 5M per, I think they should go for it. If they manage his snaps carefully, and use him as a pass rusher mostly, he could be a devastating weapon for the length of his contract.