I have been doing thesame thing lately, trying to get to tank naxx10. Currently I have: 29k hp unbuffed, 31% dodge, 32% crit, and about 29k armor (don't know for sure, and can't check it atm, sorry).
With those stats I can take somewhat damage, although I really prefer to gear up a little more before I tank the full raid. I'm a healer mainspec, and I gear up for my tanking set whenever I can. I currently have 3 pieces of T7 gained through badges and drops in naxx while healing. So may I suggest you go like me, and make a feral dps offspec, and do that first untill your gear allows to full-time tank. (Tanking is gear dependant, yes).
One thing you should know that is, if you take 3/3 Survival of the fittest, you're immune to crits and don't need anymore def rating.
This is my spec if you should be interested.
Short version: 30/30/30 ... 30k armor (maybe hard to reach to start), 30k hp, 30% dodge (and alot of crit is nice due to savage defense). (If those are to high ,a bit lower is okay too I think, if your healers can take it)
Try to start with another spec and gear up tanking set in the meanwhile. No need for crit because of 3/3 SotF.
Edited, Jun 21st 2009 2:02pm by Thorpedro