Don't know about bitstream audio (check here: http://www.gigabyte.com/products/product-page.aspx?pid=3439#sp )mrklaw said:same - I only put that motherboard in because hazaro recommended it - I'm assuming it does bitstreaming audio over HDMI (?) and optical out?
460 I have no idea - are they all the same or are some better than others? Going for a 1GB one.
And lastly, will that setup give me room to grow later. Either upping the RAM, or upgrading the GPU (can it SLI?) etc?
But the 460 can output via mini HDMI -> HDMI.
Try to get a brand that has a warranty where you are. Almost all cards are fine.
SLi yes (will run 8x/8x, but that is enough), most modern mobos support 16-32GB.
It's better to run single card and usually when you want to upgrade that is a better option than putting in another older card. However, if later on dual 460's is a good value you can run 2 with your PSU fine.