This probably belongs into A&E, but anyway:
The show did a pretty good job in covering all major plot points, so I don't think you will have any problems in keeping up with the events in A Clash Of Kings (actually I've started many rereads with that book as well). Martin throws some quick reminders about what happened in the first book into the early chapters as well, without weighting them down with too much exposition.
I still recommend that you read the first book, though. There's a lot you've missed. Lots of characterization was left out (expect some darker shades on Catelyn, for example), and especially secondary characters like Bronn have place to shine. You'll also get a proper battle (the one Tyrion misses in the show) and a much better description of how Robb defeats Jaime.
Then there's also a certain part that was completely left out, and it concerns Ned remembering Lyanna's death at various occasions, as well as some related things. I can't adequately describe what's so important about it (you have to read and figure it out for yourself), but in my opinion it's the most brilliant aspect of the whole series so far.