1. The 7870XT is somewhat equivalent to the 660Ti. 660Ti>660.If you can find a 660Ti at the same price or lower than the XT,by all means,go ahead
2. I realise it's a gaming motherboard but will you fully utilize it?My reasoning is why pay for extra when you won't be using it?
3. Personal thing,really.I've never been a fan of m ITX boards.Too dainty(rread:small) for my liking.
Gaming series motherboards are also usually made for moderate to heavy OCing and multiple GPU setups.Would you be doing either? Heavy OCs need huge cooling to make the OC stable. For a general gamer,that amount of OC is not required.
4. 8GB is plenty for gaming and running a few programs in the background.If you'll be involved in content creation(editing,rendering,video transcoding,CAD,video streaming) or running a workstation,tghen yes,more RAM is necessary so your system is not bogged down.But for those tasks,people usually get the i7 CPUs.For lightweight editing,the i5 is great.
Also,the benefit of having 2400 RAM as opposed to 1866 or even 1600 RAM does not warrant the price increase over respective RAM frequencies. A few FPS,that's about it.Won't be observable.