An i5 should be perfectly fine processor wise.
I am always confused at your insane ram usage, I have 8GB's and even when rendering i dont max it out but considering your struggling with 12GB's I'd just go for a nice 16GB kit, you don't upgrade super often so why not just go straight for it?
Well you can always get something with the same power as your GTX 460 but runs cooler so you dont lose any performance, something like a 750 ti or R9 270 would probably be perfectly fine for you if its not going to be used for any intense gaming, but say you want to play a game you wont have any problems.
Yeah, I really recommend building your own.
You get to choose what goes into it, make sure its all high quality and get the look you want all for cheaper usually (If you want I can help you by making a partlist if you give me a budget)
I would personally get 8.1 its so fast and I havent had a single problem with it, if you arent a fan of the start menu you can install something like "start is back" or similar so you have the start menu from 7
Honestly, any modern motherboards and cases will have USB 3.0 so you don't really need to worry about that.
New technology, well you can always get an SSD for the insane read/write/boot speeds but otherwise nothing comes to mind.
hmm, what kind of screen do you want? I'd reccommend something around the 24inch range with an IPS panel
if you have any specifics it would help.
Err hope that was informative, sorry for being late to the party!