pick up the self titled album (has the surface of the moon on the cover) it's pretty grooving and simple. elephant riders is also a good one to start with. those two albums are after the transition from a hardcore-type band to a jammier outfit. if you dig those, move right on down the line. jam room, pure rock fury, blast tyrant, robot hive/exodus, then from beale st. to oblivion... i am not too pumped on the last one, though. there are a few live ones in there, too.
clutch is a great band, and they're real easy to get in to. everyone i know likes clutch after i have played it for them or taken them to a show. even your girlfriend will dig clutch! all of mine have, anyway.
the earlier stuff is good, too. pretty powerful. some of the tracks on transnational speedway league still give me goosebumps all these many years later. it's just way different stuff than they've played and have been playing since 1995. i remember seeing them jam stuff out at shows in 95 or so and they weren't very good at it. they've come a long way!