[snip]

> At least under XFree86 3.3.6 (no Xv extension, I believe), the quality is
> something of a trade-off against what mplayer gives.  The picture appears
> smoother in ogle, but the contrast is lower (and/or the value of each color
> is lower).  As a result, black comes out as somewhat washed-out as dark grey,
> rather than black.
>
>
> mplayer has decidedly better colors, and I think a more correct aspect ratio.
>  (Correcting the aspect may be why mplayer's playback looks less smooth.)

[snip]

> Lastly, mplayer (at least) seems to only want to use 12 bits per pixel for
> me.  So, playing in 24 bits doesn't make its life any easier than in 16 bits
> (and, in fact, requirs 50% more data to be pumped out)

This could be what is making mplayer look brighter or "better."  If it is
indeed only throwing out 12bpp, you're looking at a severely posterized image
(i.e. 4 bits per color channel, essentially).  Some DVD players (and ATI
cards, when using hardware decoding, BTW) use 10 bits per color channel for
30bpp effective color depth.  12bpp, like a well known detergent, makes your
"whites look whiter" and your "blacks look blacker" due to the smaller
available pallette.
> The short of it: It works.  No need to upgrade OS, throw out my perfectly
> good PCI video card, or switch to XFree86 4.x.

Again, the video card will most likely have to be upgraded in the future, if
you get serious about it and/or you start noticing video artifacts.  Until
then, enjoy!  If it isn't broken, don't fix it (you won't hear me say that
again ;-).
WRT ATI vs. others, there won't be a huge difference unless you start getting
into hardware acceleration/overlay channels.  The big advantage of ATI vs.
Geforce2, for example, is the afore mentioned 10 bit/color channel decoding
capability.  Believe me, 10 bit vs. 8 bit per channel makes a _huge_
difference.  But, then again, I'm picky about my video.
In the end, I'm glad to hear you've got everything running.  Enjoy!

-- 
-Jon
 "As three unwavering bands of light, we were simple and separate and
 beautiful.  As machines, we were flabby bags of ancient plumbing and
 wiring, of rusty hinges and feeble springs.  And our
 interrelationships were Byzantine."
 - Kurt Vonnegut, Breakfast of Champions