Both the 3900XT and 3600XT are essentially just minor bump-ups from their respective predecessors. Both CPUs still come with the same number of cores and Total Cache size; the 3900XT has 12-cores, 24-threads and a 70MB L3 Cache, while the 3600XT has 6-cores, 12-threads and a 35MB L3 Cache. Even the power envelope for both CPUs are the same at 105W and 95W for the 3900XT and 3600XT, respectively.
What is different, however, are their clockspeeds. Both the 3900XT and 3600XT have received meagre increments in both their base and boost clocks. With the 3900XT supposedly running at a 4.1GHz base and 4.8GHz boost, while the 3600XT runs with a 4GHz base and 4.7GHz boost out of the box.
(Source: Reddit via Tom’s Hardware)
