If you are asking where to begin at lvl 80, heroics are the place. But before you ding 80, step into some regular versions of the instances so you get a feel for the place (and some gear to use in those heroics). I would run heroic UK a few times to get going. It is quick and rather easy.

Once you have some heroic gear, the raid progression is probably something like this (10-man), in order of easiest to hardest:

VoA (Arch) --> OS (no drakes) --> Naxx --> Maly

Be sure to get tabards for the useful reps (you did not mention your class, so I don't know what you would need) and wear them into instances. "Lvl 80" regular dungeons will also grant rep (HoL comes to mind), so don't forget them there.

Good luck!

You are probably DPS spec, but just in case, please don't jump into Heroics as healer or tank if you don't have the requisite gear, for example lvl78+ blue drops or crafted blues. No one wants to run a Heroic with a tank who doesn't have at least 535 Defense unless the healer totally outgears Heroics. If you can't afford AH gear, then try to get some Regular runs of the lvl80 dungeons -- HoL, CoS, Oculus, and UP -- if possible on your server.

Don't bother with level 70 gear. Kara gear was being replaced early in Northrend.

Start doing the argent tournament. The moment you hit 80 you have the ability to grab a weapon with those tokens.

Normal level 80 instances can be gotten into even now, so start doing them for smaller upgrades. Look up faction rep and wear a tabard, do dailies, and any quests you can for the ones with good gear for your class/spec.

Sorry I have to say this, and if I get voted down, oh well. Did you just ask if when you hit 80 you were thinking about gearing up in Kara but decided against the idea because the gear PROBABLY isn't better than 80 heroic gear? Oh man... To quote an old WoW meme...

"Kara is srs business"

You can start Argent Tournament now... You need to be lvl77, and there is a special flight master on the Dalaran Terrace who takes you there in case you dont have flying mount. Thats what I plan to do when I ding 77 :)