We're all aware of the gap that there is between the HD7770 and the HD7850, which the 650ti likes to fill.
The 7790 is set to include Amd's "Bonaire" gpu that will be based on the 28nm manufacturing process, with rumored 896 or 768 shader units/cores. It is also rumored that this card will come stock above 1Ghz. Amd is saying the card should be about 10% slower than the current 7850 model.
One interesting thing to note is that it's priced at $175 (usd). This doesn't make too much sense because the better performing 7850 is about $160 (depending on retailer http://bit.ly/14UmkL8 )
http://www.techspot.com/news/51917-amd-radeon-hd-7790-to-be-10-slower-than-7850-7990-prototype-ready.html