You're fine going into kara with a couple greens. The main thing is to have up to date weapons and all your gear enchanted, even the crappy stuff (don't put mongoose on a garbage weapon, though). The Assassination set is very good, and there are some great drops in every heroic. Grab Atlas and Atlasloot, look through the loot tables, pick out some gear, and go for it. The stat you should be most concerned with in Kara will be Attack Power, but that will change once you get into 25 mans and get better buffs.