Lord of the Rings: The Battle for Middle-Earth
I just picked this game up the other day and haven't been able to stop playing it. It's amazing! Much better than the other LOTR title put out earlier in 2004 of this same premise.
If you like Command and Conquer type of games or are into the LOTR movies, then this is a must for you. Great action and tactics in campaign mode, and there's also a multiplayer mode or a skirmish mode for scenarios of small battles.
Definitely for high-end units though. My computer isn't old at all and it has some slowdown on some of the larger battles (Helm's Deep). Keep that in mind.