scaler vs. doubler- what is the difference????

You will probably get some answers here that will suffice. Another good place to go is www.AVSFORUM.com. While you might not think many of the postings there are as thoughtful as on Audiogon, you will find many more threads along the 'lines'(oops, forgive the unintended pun) you are asking about.

A line doubler takes interlaced video, like NTSC's 480i signal, and de-interlaces it into a progressive scan 480p signal. There are HD line doublers for taking 1080i and converting it into 1080p, but I think you are looking more at NTSC input.

A scaler will convert the video feed from one resolution to another. An example would be feeding 480p into a scaler from a progressive scan DVD player or a line doubler and it sending out 1366x768 to your projector.

There has been a lot of work towards consolidating these two different things into one box that takes 480i in from a source and de-interlaces it, scales it to another resolution and outputs it to a device without having to convert from a digital signal to analog and then analog to digital again.

One other thing that is sometimes done is frame rate conversion. Typical video displays are running at 60Hz (refresh rate), but film is shot in 24 fps. To make the film stay in sync with video, they play the first frame 3 times and then the next one twice. This causes panning shots to look jerky. If you have a display device that can run different refresh rates, you could set it to be at 72Hz, thereby playing each frame 3 times and getting rid of the jerky look in pans (called judder).

A simple home theater PC can do all this very cheaply. A well built HTPC can easily outperform stand alone systems costing five-ten as much.

Based on what I've read, It's my understanding that a video scaler is by design better suited for processing video to be displayed on a fixed-pixel display device (LCD, DLP, plasma, etc.)

I run a 9" Sony 1292 on a 87x49" Studiotek 130 screen, and 480p is unwatchable (the black lines between picture lines are about as big as the lines of picture, and are clearly visible even 11' out).

If you aren't having those problems, you might want to look at your setup (optical focus, scheimpflug, astigmatism, convergance).

If you are, you probably want at least 720p. A good scaler or HTPC will give you even more flexibility here - you'll be able to pick a resolution where the scan lines just touch so you get the sharpest posible smooth, film-like image.

A scaler is also VERY useful on a CRT. One nice thing you can do is scale all your NTSC video to the sweet spot of your CRT projector.