Awesome translation of the Warhammer 40K universe. If it offered Tyranids as a playable race it would be perfect!
The basic gameplay of WH40K revolves around you capturing beacons or waypoints across the map to earn requisition points to be spend on units, upgrades, and buildings. The more territory you dominate on the map, the more you can produce. So expect to witness large scale battles in an effort to squash the opposition. The battles are breathtaking and really capture the atmosphere of the Warhammer 40K universe quite well.
Two new races are offered in Dark Crusade; the Necrons and the Tau. This is in addition to Orks, Space Marines, Eldar, Imperial, and Chaos. That's a whopping total of seven playable races that are fairly unique in play style.
The sound is really done well, and the units have some great voice overs.
If you're looking for a newer RTS to play, you really can't go wrong with the Dawn of War series. Especially considering you can get the Gold Edition, which includes Dawn of War + Winter Assault, for fairly cheap. It's a great way to get into the game.
Dark Crusade is actually a stand alone product. All races are playable in single player, however, if you want to go online, only Necrons and Tau are available unless you own Dawn of War and Winter Assault. The game is easy to recommmend and if you enjoy Dark Crusade, you'll certainly want to extend your game with the first to entries in the series.
Online play is a blast. You can play ranked and unranked games so you can get some practice before you try ladder play.
Relic is a hot RTS developer to keep an eye on. They did fantastic job on the Homeworld series, and their latest project, Company of Heroes (a WW2 themed RTS) is also stellar. You really can't go wrong with ANY game made by Relic to be honest.
I really hope they make a sequel to the Dawn of War series. And maybe this time around, they'll remember to pack in the Tyranids ;)
What are you waiting for? Go out and buy this game already!