8600GT does Bluray just fine as does the GTX260-216. For 8600GT however you need Vista as Nvidia only support Bluray on certain budget cards in Vista not XP so that could be your problem as my 8600GT will not play hi-def in XP but its flawless in Vista which is all down to Nvidia drivers not being fully featured under XP anymore. My brother uses my old 8600GT via VGA under Vista and he can watch BD + HDDVD just fine via PowerDVD7.3 so as your 9600 is a better spec card to the 8600GT it should be flawless under Vista.
In general I found Nvidia cards are better than ATI for Bluray playback as ATI keep updating their HDCP encryption keys every month yet Nvidia last around 3 months before you must upgrade the drivers to the latest.
Also that Cyberlink BD advisor is buggy as hell run it 2 different times to get different results but it nearly always wants you to have the latest gfx drivers anyway as it does an online check for this before allowing BD playback (part of the HDCP spec apparently). When you turn on your monitor/hdtv and PC also affects things as not all handshake the HDCP correctly so more often that not its best to turn on the display first then the PC.
Cyberlink still make the best BD software (unfortunately their support is not great but others are worse so best of a bad bunch) go for PowerDVD7.3 if you want HD-DVD playback as well otherwise PowerDVD9 is the latest and very stable for BD playback but not HD-DVD. Its still a real minefield to get decent HD Disc playback on the PC due to HDCP and poor software players as no-one offers a free hidef player which can playback all discs without problems although Slysoft are developing a free software player which will support all known formats but no timescale has been given.
Also be aware that spdif playback on HD-DVD has issues after PowerDVD7.2 as Cyberlink broke the lipsync so it lags after around 15 mins. If HD-DVD is not something you have then ignore otherwise you need to install PDVD7.2 and rename a file to prevent the software asking to update itself and refusing to play any media!
Forget WinDVD9 as the PQ is poor vs PDVD9 and HD-DVD is broken on several discs as the encoder cannot handle the material so the PQ breaks up into pixellated blocks.
Same for Arcsoft Total Media. Most versions have issues with hidef playback and its not cheap so forget that as their support is much worse than Cyberlink (if thats even possible!!).
If your using a high end soundcard in your PC you may also find additional problems as not all pass the audio correctly as some use dongles which break the HDCP chain & or the software player will not recognise it!
Apart from all that

enjoy as PC provides a superior PQ to any standalone hidef players