It * could * be that your TV-chip is not supported by the Nvidia drivers. Myself I have a GeForce FX 5700 Ultra ViVo, manufactured by Point of View. The TV-chip on this one is the Philips SAA7114E. It works great in Windows XP with the 53.03 drivers, but not so in Linux. After playing around with several driver and kernel versions I came to the following discovery:
* in my case TV-out gives the same results (bad results, that is) whether working with kernels 2.4.x or 2.6.x.
* AGP is not the issue here: enables/disabling Fast Writes or SBA does not make a difference, neither does using the Nvidia AGP driver or AGPGART from the kernel source
* funnily, I do get TV-out with the 4620 (beta) drivers, but NOT with 4496 or 5328 drivers! The picture with the 4620 drivers is of * very * poor quality, however. In my /var/log/XFree86.0.log it says
(--) NVIDIA(0): Detected TV Encoder: Unknown NVIDIA
with the 5328 drivers, while with the 4620 drivers it says
(--) NVIDIA(0): Detected TV Encoder: NVIDIA
I haven't tried the latest (5336) drivers yet... maybe they'll fix things but I'm not counting on it because my particular TV-chip doesn't seem to be very common.
By the way, I used to have an ASUS V7100 Geforce2 with BT869 chip on-board and it worked perfectly, so my problem is not related to anything other (cables, configuration settings) than what I've already mentioned.
You can check http://tvtool.info/cardbase.htm
to see if maybe your card is listed here and find out which TV-chip you've got on your card. An alternative would be to take the card out and take a peek yourself :-)
I've attached my complete xf86 log file, in case anyone thinks he or she can find anything useful in there.