The 734 has a few other proprietary techs over the 733 (Dark Detailer, Deepfield Imager, Perfect Tint). I think these help with overall picture quality, mostly because they allow better control over color (Perfect Tint). However, the other techs I actually prefer on, unlike most other manufacturer techs (Samsungs DNiE and others like it).
I don't feel the 833 is worth the extra money over the 734, unless you're really affected by judder, in which case the 120hz refresh of the 833 may be worth it.
Here's a link to another discussion on the Mits DLPs.
I'd heard about the whining on some people's Mits DLP, but mine has none of that (and I suspect that most sets do not have a problem). The biggest (really, only) problem that I had with my 65734 was some slight pincushioning. Mostly noticeable with football games with all the yard lines. However, I was able to mostly correct it with a quick trip in the service menu; it wasn't that bad to begin with and is now mostly gone.
All in all, and excellent set in my opinion.