Before we start, there's two things you should remember: One, I'm pretty much following the logic I laid down in the Death Knight statistics primer a few weeks back, so if you want to know why I picked a certain item, the answer is probably there, and two, whatever you do, remember to hit 540 defense skill. You'll need that to survive against the bosses.
Heroic Gundrak is your best bet to grab a Helm. The Ground Tremor Helm off Moobrabi has pretty much everything you could want from a tanking Helm, including a very nice chunk of defense and 2 gems slots, one red, one meta, perfect for, say, a Champion's Monarch Topaz and an Austere Earthsiege Diamond.
If you can stand to lose some defense and feel like you have enough hit rating, the Arcane-Shield Helm from Telestra in Heroic Nexus is also a good option. Despite having much less defense rating and no hit rating, It has a more strength and a nice chunk of dodge, giving it much more avoidance. It also has a metagem slot, as well as a blue gem slot that's perfect for a Solid Sky Sapphire or an Enduring Forest Emerald.
Of course, you'll also want to grab the Arcanum of the Stalwart Protector from the Argent Crusade to finish your helm off, if you haven't already.
You can probably get away with keeping your Pauldrons of Reconnaisance from Halls of Stone going into Naxxramas, but if you're looking shed some defense in favor of some more avoidance and stamina, go to Heroic Halls of Lightning. General Bjarngrim drops the Iron Dwarf Smith Pauldrons, which are handily superior in every way except for a bit less defense rating. Be sure to grab a Pinnacle enchant for your shoulders from the Sons of Hodir or inscription too.
The Breastplate of the Solemn Council is a solid choice even for 10-man Naxxramas Tanking, and you probably won't go wrong keeping it around. But if you just have something against grinding revered with the Wyrmrest Accord, the Sun Emblazoned Chestplate off Sjonnir the Ironshaper in Heroic Halls of Stone is a nice sidegrade that may prove a slight bit better depending on your needs, mostly because of the blue gem socket.
For a Chest enchant, Greater Defense is a great way to shore up your defense and allow you take lower defense rating higher stamina and avoidance gear in other slots.
Your best bet for a Wrist slot will be downing Herald Volajz in Heroic Ahn'kahet for the Bracers of the Herald. They're not only nice and purple, but they have a nice outlay of defense, stamina, strength, and parry rating. Of course, if Volajz is refusing to drop your bracers, you can get the somewhat inferior but still pretty nice Bindings of Dark Will off Salramm the Fleshcrafter in Heroic Culling of Stratholme.
Your best bet for an enchant for your wrist is probably to wait for 3.0.8 to drop and for that 40 stamina wrist enchant to go live.
The Gauntlets of Vigilance remain a nice, easy to get quest reward that hold up well with the big boys, and it's from a highly enjoyable quest chain at that, but if you're looking to beef up your hand slot, here's a few other ideas.
The Fireproven Gauntlets from Kirin Tor exalted remain the only loot-related reason for a Death Knight to grind exalted with Kirin Tor, but they're a very good reason. Dodge rating makes up for the loss of agility on the Gauntlets of Vigilance, and they have more armor, strength, stamina, and defense rating besides.
If you don't feel like you have time to grind exalted with the Kirin Tor, you could also try running Heroic Violet Hold a few times. Ichoron, one of the random bosses in there, can drop the Gauntlets of the Water Revenant, which are sort of a weaker version of the Fireproven Gauntlets, but with more defense and hit rating instead of dodge. They'll be a solid choice until you can get an upgrade.
Edit: A Couple readers pointed out that I forgot to mention the Horn-Tipped Gauntlets from Heroic Gundrak. These are a very nice set of tanking gloves, especially since they include a very large amount of expertise.
For a Glove enchant, it shouldn't take you too long to farm the eternals for Armsman.