Which NLE/Cineform combo would give:
1) the closest to real time preview
2) the fastest rendering

assuming the same footage with similar effects.

I currently use Vegas 8 Pro with NEO HDV and am considering getting Aspect HD to use with PPro CS4.

Currently Aspect (or Prospect HD) plus CS3 is the fastest, CS4 support is coming. CS4 doesn't yet have the API for third party accelerators (coming from Adobe in CS4 v4.0.1 shortly.)

David, can you give a sense of the relative increase in rendering speed using PPro CS3/Aspect vs. Vegas Pro 8/HEO HDV?

To answer my own question, I downloaded the trial of Aspect HD and got these rendering times with a 10-second Cineform HDV clip (High quality) with a heavy Magic Bullet Looks effect applied:

Sony Vegas 8/NEO HDV: 244 seconds
PPro CS3/NEO HDV: 220 seconds (about 10% faster than Vegas)
PPro CS3/Aspect HD: 187 seconds (about 23% faster than Vegas)

I've got an Intel 1.8 Ghz Core 2 Duo, nVidia 8600 card, raid 0 video drives.

Can anyone comment on these relative performance numbers--i.e. are they consistent with your experience?