I was going to go with Citadels, Carc, and Roborally. :\
Maybe Galaxy Trucker? Build space ships that get destroyed by the game's random events. Your goal is to get destroyed the least by building quickly but still making a well engineered ship. Tile placey, but timed and simultaneous, goes decently quick.
Same designer: Dungeon Lords - a heavier game, and only plays with 2-4, but it's pretty much the old PC game Dungeon Keeper in board game form. You guys each build dungeons that get explored by heroes. Use traps/monsters to kill the heroes and defend your evil lair.
Or maybe, since you're an RPG group, a dungeon crawl game? Only one I have much experience with is Descent, which is long, has a huge set up time, and is complicated, but is pretty fun. One player is the Overlord (an adversarial DM), and the rest are the party. One of the expansions adds a campaign mode to carry over sessions. There's other dungeon crawls as well, including a couple D&D branded ones. There's also other non-dungeon crawly co-op stuff that you guys may want to do... Arkham Horror maybe? This is not a side of board gaming I do much of :\
I honestly think Astrolad's suggestion of Citadels is probably the best one for your group. Cheap, light, plays well with any number, and offers GREAT screwage opportunities.