Green Man Gaming has a sale on a few bundles. The only one that's really interesting is their bundle of Civilization V and Tropico 3 for $36.37. I've never used GMG, and I've heard some nasty things about their client. That said, the site says that Civ 5 requires Steam, so you'll probably be...
Why does it matter? The Impulse serial will work with the trial version of Warband you download here:
http://www.taleworlds.com/main.aspx?dir=download.aspx?type=2
so you don't need either the Steam or the Impulse client.
That link doesn't include Civ 5 'cause it isn't out yet. Civ 5 includes Steamworks, so your retail serial key will work in Steam whenever the game is released. In fact, the game will probably require Steam, regardless of how you buy the game. Shacknews says as much...