See you round guys!
If you've got a good power supply and a large enough case for a full size card, the 750Ti is not a good choice. It'll be a noticeable upgrade, but there are better cards for the price. The next step up from a 750Ti isn't necessarily a 760. There are a lot of cards that fall between those two price points.
As for the RAM, how about getting 4 more gigs now and wait for the predicted price drop to grab the other 8?
Can't answer the graphics card question, but why are you upgrading to 16gb? You'll barely need 8gb these days, unless you're using some really taxing software. If your rig is for gaming stick to 8gb
It would be noticeable... but the jump from the 750ti to the 760 is a very dramatic one. I would go with the best single card you are willing to pay for.
What are the rest of your system specs?