The small arm boards are wonderful for games, entertainment and hand-crafted appliances of all sorts. Pick one supported by excellent Armbian or use a recent Raspi. The boards you own are just fine for that.
There is no such thing as "General PC" but you state requirements as
- truly silent and very, very low power
- desktop for my research/writing.
- DTP and graphics
- fixed to the rear of a monitor
- accelerated video and reliable sound - cost IS a consideration
Your OPI PC is cheap, silent, low power with accelerated video and reliable sound when hooked up to HDMI-monitor with speakers. You can also use it for research/writing (even DTP and graphics) , but don't expect stellar performance.
You do get quite pleasing performance by using OPI PC ( tested on smaller sibling OPI ONE ) as a capable thinclient accessing a physical (cheap old linux-converted notebook or PC ) or virtual ( virtual private server possibly living in the clowd ) linux server running suitable powerful software, keeping vital data centralized and universally accessible. Does this sound like what you want ?
The recipe for your "general PC" is explained at http://forum.armbian.com/index.php/topic/1044-remote-desktop-fun-with-armbian-505-on-opi-one/