|I can't answer all those questions but to answer the HDMI one:|
"So I take it downstairs to hook to my 1080p RPTV, a JVC HD-56FH97 and things are not so great. At first sound output did not work via HDMI, but after updating the Realtek HD audio drivers, the nForce drivers and the Foreceware drivers (none of which were available through HP, they have not updated the original drivers for the s3600z on their support site), and then checking my settings again to switch the default output device to HDMI, now the sound is finally working via HDMI."
Yes, you should be able to get audio from HDMI. My dad's TV had to enable something, i can't remember, which then got the DVD player working with the audio. I read that HDMI from PC to TV only gets you stereo channels and not 5.1.
Depending on the onboard audio, the quality might not be there. Also, depending on the sound system you have, it might not matter. TV speakers for example. If you have a Bose system, then you might want to get a better sound card.
For the rest of the questions, i don't know. I don't have Vista. The 5 gigs of ram, that should be plenty just to have sound on your TV.
AMD64 X2 4200 @2.5Ghz
Corsair 3Gig Dual-Channel 2x512 2x1gig
MSI Radeon x800 PCI-E
Seagate 160Gig SATA
Seagate 320Gig SATA