So you've been running Heroics for a while now, right? Do you have 40 emblems? Okay, buy the Waistguard of Living Iron. Trust me. It's really for the best. Then you can get an Eternal Belt Buckle on that thing, stick a few gems in, and you'll probably be set even up through 25-man Kel'thuzad, it's just that good.
This is another slot where you can probably get away with your reputation gear. The Special Issue Legplates should be a solid choice for a beginning Naxx tank, especially with the gem slot and all. However, if you want to go the extra mile for the upgrade, try taking down Cyanigosa in Heroic Violet Hold.She drops the Bolstered Legplates, which are essentially a turbo-charged version of the Special Issue Legplates, right down to the red gem slot.
Be sure to slap on a Frosthide Leg Armor kit on your pants before your first Naxx run while you're at it.
Once again, you probably won't be too bad off sticking with the Sabatons of Draconic Vigor from Revered Wyrmrest Accord, but if you can stand to lose some defense rating and hit rating and feel like you need more avoidance and stamina, consider the Greaves of Ancient Evil off Heroic Herald Volazj. They're a nice upgrade from the sabatons, with more strength and stamina and some dodge rating in exchange for less defense rating and no hit rating.
A good solid and relatively cheap enchantment to put on those boots would probably be the Greater Fortitude Enchant. You might also consider Tuskarr's Vitality if you don't have Unholy Presence, but it's pretty restrictively expensive right now.
Ok, so you bought the belt, right? Do you have another 25 emblems saved up? Great, go ahead and grab the Chained Military Gorget. It has plenty of defense, dodge, and hit rating in addition to a healthy service of strength and stamina.
The Cloak of Peaceful Resolutions from honored Wyrmrest Accord is probably going to remain your best bet for tanking in Naxxramas. If you're looking for an alternate look, though, you might consider the Cloak of the Enemy from Heroic Nexus. It's essentially a sidegrade, but it does have a few extra points of armor and stamina, and it definitely looks better.
If you're willing to spend a bit of extra gold to get geared up for tanking, strongly consider purchasing a Titanium Earthguard Ring. It really is a pretty amazing tanking ring, plus it has a very nice blue socket waiting for a gem.
For drop options, check out the Woeful Band from the Maiden of Grief in Heroic Halls of Stone, or the Band of Torture from Heroic Oculus, both of which provide a good bit of defense rating and some sizable avoidance and stamina as well. If you feel like you could some extra expertise, you could also dig all the way back to normal Gundrak for Gal'darah's Signet, which compare surprisingly favorable to your heroic options overall.
In yet another instance where you'll want to leave behind the Heroics for a bit, consider running normal Halls of Lightning for a bit. The Seal of the Pantheon is the only 5-man defense trinket you'll find in Northrend, and it's pretty good anyway.
Beyond that you'll probably want to shoot for the Essence of Gossamer, which drops off Hadronox in Heroic Azjol-Nerub. While it has no defense, it does have massive amounts of stamina and a nice damage absorption proc - which luckily doesn't worry us, since we don't need to get hit to generate runic power.
For weaponry, Consider buying or gathering the mats for a Titansteel Destroyer. The massive amounts of strength and hit rating will provide both plenty of threat and some extra parry rating.
If you don't want to spend the money on the Destroyer, you have a couple other options. The Sword of Justice from Heroic Halls of Stone has plenty of strength and stamina and a gem slot besides, one of the only 2-handed weapons in Northrend that even bothered including a gem slot.
The best dropped tank weapon pre-Naxxramas, though, is probably the Colossal Skull-Clad Cleaver off Loken in Heroic Halls of Lightning. Lots of strength and agility equals lots of threat and avoidance, and Orcs may even prefer it to the Titansteel Destroyer due to their racial bonus.
As far as which enchant to choose, for now it's probably Rune of Swordshattering, but come 3.0.8, that's going to be a bit more difficult to choose. Rune of the Stoneskin Gargoyle's 25 defense skill (not rating) and +2% stamina will open a lot of new gearing avenues, and you'll probably have to decide if you want the avoidance from Swordshattering, or the freedom to be more flexible with your gear from Stoneskin Gargoyle. In the end, it may be that you'll make up any avoidance gap from eschewing Rune of Swordshattering via the extra avoidance on your gear and that extra 2% stamina from the enchant.
Still, don't discount Swordshattering completely, especially if you're a Blood tank with Spell Deflection. Once you're flush with defense gear, you may end up giving it a second look.