1) If you can build a comp, building a server is almost as easy since everything is geared toward reliability ... PSU x 2, CPU x 2, increased RAM, more fans (noisy) that run at higher RPM. The big difference is RAID setup, but from your correspondence I think your RAID 5 is more than sufficient
2) Since this is mainly a home use HTPC like, the I/O demand will not be that significant
... A) Dual 250GB SSD RAID setup for OS will give you speed & reliability
... B) For what you're doing at home, increasing cache is a lost leader & not a good ROI. Save money and use it on other equipment
... C) 250GB SSD x 2 = $160 - $260
... D) 8-10 TB HD x 3 = $950 - $1500
3) Audio Components (for main TV primarily)...
... A) Receiver that can handle at least 7.1 dts, HDMA, True HD Dolby & etc ... as well as Bluetooth. There are other nuances but this is not the forum to go into details
... B) Speaker system w/ subwoofer ... sorry, but BOSE doesn't count
... C) A dedicated amplifier to drive these components
... D) For other TV, I would get a decent soundbar w/ blue tooth subwoofer ie. Samsung HW450 series ... approx. $300 for decent audio effects at least
... E) For best result, use the TOS link (fiber optic) to connect audio to components

Good luck.