Ok, I have Blur and NR checked first (it's gonna kill your cpu otherwise). In there I have gradual and denoise3d set to the settings I specified before. I leave 'process whole image' unchecked here because there's no reason to denoise black bars (waste of cpu).
Next is resize. I resize to 1440x960 using lanczos (and output to my tv at 1440x960i) and with 'interlaced' checked (since it's for dvds). Keep 'process whole image' checked here, you don't want ffdshow to crop the black bars and resize only the used area because then scaling would be off.
You really want to use newest version of ffdshow found here http://athos.leffe.dnsalias.com/.
In the older versions I could definitely tell that 'spline' was better than lanczos, but with the new version lanczos seems every bit as good (without using as much cpu).
For sharpening I set chroma and luma resize sharpening to 1.60. This may seem excessive, but on my tv I set it's native (i.e crappy) sharpening completely turned off. Also I disabled velocity scan modulation, so my set doesn't sharpen AT ALL.
However you'll generally want to resize to your displays native resolution. If you have a plasma that's only capable of 856x480 (or fill in blank) then that's what you should resize to. If you don't it's gonna scale it down to that size anyway, and it won't be using a method as good as lanczos. Also you'll wanna output at that resolution from your pc! Now as I said I use 1440x960i, which is not the native max resolution of my HDTV, but I use DVI and 1920x1080i didn't fit well at all and 1440x960 did and happens to be a perfect resolution for scaling dvds.
One more tip. I use resize sharpening, but this method is not as good if you are resizing to smaller dimensions, in this case you may want to stick with descaler's sharpening method with is also pretty darn good.
Anyway, this is what I use. Keep in mind I have a crazy fast CPU. It's a 3.2 with hyperthreading enabled. It's cool though, even with this all enabled my cpu usage is only 35-40% for dvds!