It's easy when you have the right hardware. I struggled more than I want to admit without any real success to make my ATI Radeon 7500 All-in-Wonder card do NTSC video out from MythTV.
I was frustrated enough that I was pricing nVidia cards on the web, knowing that their driver would "just work" despite being hideously non-free. To the rescue came a co-worker, who pointed me to Thomas Winischhofer's Linux and SiS site, which suggested that I could have a fully open source solution for NTSC video output for cheap.

And it works!

I bought an ECS AG315P/TV card from newegg.com, which is fast becoming my favorite place to buy stuff like this. For the princely sum of $29, I got an AGP video card using the SiS 315 controller and an SiS 301 video bridge... that auto-detects the presence of a load on the RCA composite output at boot time, and gives me full BIOS video support on the TV output!

I didn't even need to load up Thomas' drivers for X, the stock Debian 4.3.0-0pre1v1 from experimental "just worked" with the following device selection. The option tells the card to assign the one available set of overlay support registers (which MythTV needs for video playback) to the TV output instead of the default VGA output. That's it!

Section "Device"
        Identifier      "SiS315PRO"
        Driver          "sis"

        Option          "XvOnCRT2"              "true"
EndSection

Wish I'd known about this before I went do the ATI path... and by the way, I decided I really don't like the feel of the ATI UHF remote control, so I bought some IR modules from Radio Shack and am building up lirc receivers... but more about that another day...