At this point while FM2+ is guranteed an upgrade to the Kaveri core, AM3 doesn't have a real release schedule of any Steamroller based next gen CPU as of yet. Mainly because the AM3s are based out of Global Foundries and their shrink processes are stuck at 32nm.
I'd wait till they release the A10s based on the Kaveri core next month and buy a fully compatible mobo then.
DDR4 is indeed around the corner but consumer grade release would probably be no earlier than next summer and prices should start coming down end of 2014, however I could almost predict that the price per GB won't come down to the present DDR3 level for a while.
FX-8120, Sabertooth 990fx, Radeon HD 7790, Corsair 8gb, Plextor 5P 128gb
+ A8-6600K(got it free at E3!)