What does memory controller capacity have to do with overall performance, except in rare cases where the memory limits of your GPU are exceeded?
That being said, a Phenom II x6 1055t is not a super fast performer, wasn't even when it was new, and will definitely leave a lot of the 980 Ti's performance on the table. However, since all you're asking from your system is that it stream some online content and play Diablo III : RoS, and the requirements for that game seem comparatively low, you should be more than fine until you put together your new system. Also, if you're running a 60 Hz display, there's a good possibility that you won't even be missing any of the extra performance that you won't be achieving in games by using the older CPU.