Please help, 7800 GS with LCD projection HDTV

I have a Sony KDF-E55A20 TV that I'm using as a computer monitor.

When I use an analog signal converter to convert my VGA output to Component,
it looks pretty good, but the picture doesn't fill the screen.

When I use a DVI to HDMI cable co connect the nVidia's DVI output to my TV's
HDMI input, it fills the whole screen, and the colors are much better,
however the screen is "pixilated" if that's the right word--if you've ever
seen an LCD screen set to a non-native resolution, you know what I mean.

I looked up the native resolution of the screen, and Sony claims it's 1366 x
768. When I try to set that as a custom resolution in ForceWare Release 90
(Version: 91.31 Release Date: June 23, 2006) it will not even test the
resolution, it just says "invalid resolution."

UPDATE: If I use PowerStrip to set the resolution manually, I get a very
small picture in the center of the screen that "vibrates" or moves up and
down slightly very quickly.


Here's Sony's specs on the TV:
Video
Native Resolution: 720p
Display Resolution: 1366 x 768
Vertical Size: +5 to -5
Vertical Center: +5 to -5


How can I get the picture to look as good as it does with the analog
converter box as far as smooth lines? Please, any help would be
appreciated. I'm researching this problem myself (found & tried PowerStrip)
but am only getting more frustrated...