I'm running Windows 7 with an i5 chip and 8MB of RAM. I'm not seeing the same issues unless dealing with very large files from the D800 - and even then not to the same extent. I do have an upgraded graphics card.
<1) program crashes if I try to CONVERT more than one image from RAW to Jpeg. Sometimes even with 1 image. And, it takes forever.> I'm not seeing this issue. I'm seeing conversion times of around 2 seconds per file for D600/D800 images - faster for smaller files. You may get slightly better performance if you save a file before converting it when you are concurrently applying edits. It also can be slow if you have prior steps that are not finished processing.
<2) Nagging tendency to file uploads in folders I didn't choose. And which are named with old names that I've changed multiple times. Arghhh!> This is completely user controlled - in Nikon Transfer. You might take a look at workflow. I create a new folder with the date for every upload.
<3) Sluggish navigating from folder to folder.> I see this too. It takes time to render thumbnails and large folders make it much slower. I try to keep folders at 500 images and smaller - ideally 250-300 images. That's basically one card per folder.
<4) General slowness. Even in renaming.> I have not seen this, but I don't rename files in batches very often. Typically I see individual files renamed instantly. There was a major step to faster speed with the upgrade early this year, but it is partially offset by larger files from the D600 and D800.