It all depends, depends on what the actual video processor in each is.
Lumagen makes among the best video processors available, and is likely better than most projectors or Blu-ray players, but if you've got a really high end projector (Sim2, etc) it may not be an improvement. Likewise with an Oppo, I'm not sure what they use, but I think it's pretty good.
HTPCs, depends on the software, it can range from not possible, to terrible, to best of the best (madVR).
Of course there's only one of these, the Lumagen (maybe some other external processors) that can stretch anything, regardless of source. Displays and players tend to disable (or be incapable of) stretch for 3D for example. Retail PC software I think can't stretch, but "free" software can, but that requires ripping/decrypting Blu-rays. I don't think streamers or PCs can stretch streaming content (Netflix, etc). A Lumagen can stretch all of that.