I'm also thinking about moving from Nuendo 4 to PT8LE (for some reasons that will make a long posting), but my advice: Steer away from HD at this moment. The big problem with PT is their current lack of PDC, strange, cos its really simple to implement (in LE).
The reason I can see is the HD cards beeing outdated in terms of available onboard memory, not able to create enough big buffers to handle all the plugin latency.
Digi ofcourse knows this, so doesn't include PDC in LE alone, cos the HD sales would fall and seriously harm the HD rep.
So they will build new HDcards in the near future, with enough onboard memory to handle long convolution reverbs and FFT eq's. Maybe even AIR sample-based instruments running straight from HD DSP's... That would absolute kill everything available, DDR2 is dirt cheap anyways.
But those cards aren't here yet, and the call for PDC becomes so loud Digi can't wait that long with a new HD solution.
Stick with LE for now, upgrade to HD in the near future, let Digi iron things out first.