I've got a dual-core Athlon in a similar 990 board, and my 750 Ti still produce slightly less hashrate than the same cards in z87 rigs with dual-core Pentium G3xxx, so you want at least a dual-core Athlon working @ ~3 GHz. If you'll find a cheap Phenom x2 (a lot of them have the 3rd or/and 4th core stable, so they can be unlocked and become a triple-/quad-core), or Athlon x3/x4, that would be even better. I suppose what matters here is that you need to have at least 2 cores and you need something around 3+ GHz (for AMD cpus). Not sure about the cache requirements, so better be safe I guess and look for Phenoms first.