Thank You !! I thought nobody own this hardware. Glad you have one
1. Go for GT610. I was in dilemma just like you. Ordered Asus silent GT610. It just works. 20$ more Worth spending.
2. Yeah! Tried Yosemite Everything is working good. I recommend fresh install both OS if you are dual booting, because Yosemite dreps partitions.
3. Nope they are almost same. Just different clock-speeds. You wont be facing any problem.
4. For Audio and Network Stick with Multibeast 5.5.5. For everything else you can use the latest one.
DH55TC and DH55HC are pretty much same board. Technically only difference is PCI slots. That won't be a problem.
I forgot to update the post. Front panel audio wont work if you install ALC888.
To make it work you have to install
Drivers-->Audio-->Universal-->voodooHDA v2.7.3
But it produce hisss sound and also cause kernel panic on boot [Most of the time].
That is the only problem.
Go ahead !!