Gaming power isn't really needed for a DVR so the 3850 would be overkill; I believe even the Radeon HD 3450 has the same video decoding abilities as the 3850.
Nvidia's pretty much abandoned the AGP market, so you'd probably have to go with an AMD/ATi product if you wanted a reasonably new graphics card. While newegg does show an Asus Radeon HD 3450
, the comments seem to show people having some driver trouble with the card. I'd suggest you choose between the two 3650s (Sapphire's card
or HIS Hightech's card
). They have the full Avivo HD video decoding abilities plus can do a bit of lower resolution gaming should you want to play a game or two on your TV (recently I had some fun replaying KotOR I and II on my TV using my DVR). If you can use HDMI go with the HIS card since it comes with a DVI to HDMI connector, but if you're using component go with the Sapphire card.