Dawn of War isn't in the games database, and I have never tried it nor do I own the game, so I don't know.
I also don't know about the wireless.
But yeah, CS:S runs in full screen, and I dunno about Xfire, never used it.
I know you can chat with your Xfire thing with gaim, but I don't know much else about it. Do you really need it? I mean, what does it do?