COD2 is okay for MP if you find a good modded server with ample amount of people, or at least ones that are skilled and fun to play with. The many servers and their mods make for many interesting playing experiences. The best mods are the realism, since you can actually kill someone with a shot to the chest. After a while it can get booooorrrrrringggggg. Try BF2 if you're up to it. It has some good mods coming out plus good expansion pack(s) not sure about the new one tho.

As for RTS try CoH or the new Dawn of War: Black Dark(whatever it is) Crusade. I think it is stand-alone so you don't have to buy DoW just to get it. Battlestations Midway just released its' demo along with Supreme Commander. You also might wanna try MadMinute Games, Inc.'s Take Command 2nd Manassas, it is supposedly the best American Civil War game out there.

These are some off of the top of my head, hope it helps.