Well try to get some TBC 70 gear as much as possible, but wearing something is better than wearing nothing i guess Goal should be TBC gear, but for now for instance I'm still wearing some heirlooms untill I get better.
If it helps you out, you're allowed to use Northrend greens on TBC instances, though I would certainly not recommend it, and frankly, don't see the point. But hey, to each his own.
However, for raids proper, you are strictly restricted to TBC gear.
Question came up, the other day. That includes no heirlooms of any kind.
Heirlooms are pointless for level capped characters anyway, and they're generally on par with the equivalent level blue items, which should be coming from the heroix.