Glad to see you have done your homework.
Your choice of filters is to be commended. Dscaler sharpen and resize sharpen work in very different ways and compliment each other very nicely.
Descaler sharpen is great for bringing out fine details, unfortunately it also brings out noise so that you should not use it to excess.
Resize sharpen improves the 3D look of the image and does not noticeably increase noise. The two together work wonders.
Gradual denoise, after resize is a good choice for your level of hardware. Used in moderation it helps to remove noise without degrading the image too much.
Denoise3D used before resize works better but is very demanding on CPU.
Noise reduction is of coarse optional, and not required.
Your FFDShow settings should be subtle and not immediately obvious for best results.
Many people make the mistake of using aggressive settings and then complain that they don't like FFDShow.
As for your shuddering problems, I suggest you try to overclock your system before you replace your RAM. Most RAM can run much faster then specification.
By overclocking you increase RAM and CPU speed. With any luck you should be able to get the RAM running at about 146Mhz and the CPU near 3.3G. This would definitely help. Better RAM would allow about 180Mhz memory speed with a 4/5 divider for the same 3.3G CPU speed and would be even faster.
I used to use a 3.06 P4 @ 2.26 with RAM running at about 180Mhz and it could definitely run the filter combination you want to use.
I tested my 3.06 P4 Notebook with 166Mhz RAM and it can run your settings (just).
512Meg is more then enough RAM for video and DVD playback. Even 256Meg is fine.
Make certain that you are using overlay for output. VMR9 will be much to demanding on your system.
I recomend FFDShow 20032709.
The ffdshow resize-sharpen dude.