Ok, I'm probably missing something really obvious but I can't see it.
I'm constantly getting a "Failed to Reinit Video" when I enable XvMC.
When i use standard MPEG2 decoder it works (with heavy CPU load and not very well in HD)
Any ideas anyone?

And to add some background:
I had my KnoppMyth (R5F27) setup with an ATI Radeon Card, but since it doesn't support hardware acceleration under Linux I thought I'd get something else.
Found a cheap FX 5200 on ebay and checked that it was supported.
Have now plugged it in, updated drivers, updated /etc/X11/xorg.conf:
Section "Device"
Identifier "Card0"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BoardName "GeForce FX 5200"
Option "RenderAccel" "true"
Option "NVAGP" "1"
Option "UseEvents" "True"
Option "XvmcUsesTextures" "false"
EndSection

I've tried with a few different NVIDIA drivers (since someone suggested that the 100.14.19 was buggy), i.e. 100.14.19, 100.14.11 and currently running 1.0.8756.

/Fredrik

XvMC "just works" for me with a cheap FX5200 card. I'm not using any options in the Device section for Card0.

Have you seen these threads?
http://knoppmyth.net/phpBB2/viewtopic.php?t=16748
http://knoppmyth.net/phpBB2/viewtopic.php?t=16557

Yeah I've tried that.
i seem to load the drivers fine, and the display is ok. The problem is that when I try to enable HW acceleration with XvMC it comes up with the OSD info display on a black screen then goes over toan error saying "Failed to Reinit Video" on a blue screen with the choice to "Return to menu". Clicking the Return to menu gives me another error saying "Error was encountered while displaying video." (thanks I realised that) with yet another "Return to menu" choice. This brings me back to the main Myth FrontEnd.

I've only seen one other instance of this error, and that time the guy fixed it by assigning m ore memory to it, from 16MB to 64 MB. First of all I've got 128 MB builtin so that should be enough, secondly I don't know how to assign more memory to it?

And another weird things, if I go to the tvguide and select to record a program it does it (which is not so weird since it has nothing to do with the frontend) but when I go to the Media Library->Watch recordings i get a little preview picture of the recording and i can watch the recording, but it uses software decoding not hw.

Is there anything interesting in the X or MythTV frontend logs when that happens?
http://www.knoppmythwiki.org/index.php?page=CheckingLogFiles

Sure is, I've certainly done some things wrong while messing with this. For one I've got a driver version mismatch so I'll start trying to sort that out.

2008-01-17 14:53:54.506 TV: Attempting to change from None to WatchingLiveTV
2008-01-17 14:53:54.508 Using protocol version 31
2008-01-17 14:53:54.955 NVP: Disabling Audio, params(-1,2,44100)
2008-01-17 14:53:54.958 VideoOutputXv: XvMCTex: Init failed
2008-01-17 14:53:54.959 VideoOutputXv: XVideo Adaptor Name: 'NV17 Video Overlay'
2008-01-17 14:53:55.619 TV: Changing from None to WatchingLiveTV
2008-01-17 14:53:55.620 Using realtime priority.
Xlib: extension "GLX" missing on display ":0.0".
2008-01-17 14:53:55.723 Video timing method: RTC
[mpegts @ 0xb73a27f0]Parser not found for Codec Id: 94211 !
2008-01-17 14:53:58.460 VideoOutputXv: XvMCTex: Init failed
2008-01-17 14:53:58.472 VideoOutputXv: XvMC Adaptor Name: 'NV17 Video Overlay'
Error: API mismatch: this NVIDIA driver component has version
100.14.11, but the NVIDIA kernel module's version does not match.
Please make sure that the kernel module and all NVIDIA driver
components have the same version.
2008-01-17 14:53:58.473 VideoOutputXv Error: Unable to create XvMC Context, status(11): BadAlloc
2008-01-17 14:53:58.474 VideoOutputXv Error: Failed to create XvMC Buffers.
2008-01-17 14:53:58.474 VideoOutputXv: Falling back to X11 video output over a network socket.
*** May be very slow ***
2008-01-17 14:53:58.474 VideoOutputXv Error: XCreateImage failed: XJ_disp(0x8a86fe0) visual(0x8e0b528)
XJ_depth(24) WxH(1024x768) bpl(3072)
2008-01-17 14:53:58.475 VideoOutputXv Error: Failed to create X buffers.
2008-01-17 14:53:58.475 VideoOutputXv Error: Failed to get any video output Exiting playback.
2008-01-17 14:53:58.475 VideoOutputXv Error: InputChanged(): Failed to recreate buffers
2008-01-17 14:53:58.475 ReinitVideo(): videoOutput->IsErrored()
2008-01-17 14:53:58.620 TV: Attempting to change from WatchingLiveTV to None
2008-01-17 14:53:59.223 AFD: Opened codec 0x8a157e0, id(MPEG2VIDEO_XVMC) type(Video)
2008-01-17 14:53:59.224 AFD: Opened codec 0x8a0dd50, id(MP3) type(Audio)
2008-01-17 14:53:59.224 AFD: Opened codec 0x8a33550, id(AC3) type(Audio)
2008-01-17 14:53:59.225 Couldn't open decoder for: /myth/tv/1010_20080117145357.mpg
2008-01-17 14:53:59.225 NVP, Error: JumpToProgram failed.
2008-01-17 14:53:59.225 NVP, Error: Unknown error, exiting decoder
2008-01-17 14:54:01.290 TV: Changing from WatchingLiveTV to None

Ooooops, broke it real good now.
Boot up fine to begin with, then black screens. Won't let me access through SSH either. Maybe a nice clean install

Solved!!
Still not quite sure what the different drivers vs kernels is but by running both the nvidia supplied shell installer and the knoppmyth installer of the same driver version did the trick!
Thanks guys!