Eugene, a few points to maybe consider related to PD:
1) After testing about 15 different procs over the last year, to me, PD user experience for tasks that are CPU intensive pretty much mimics basic CPU performance charts. You can get a good feel of that here
http://www.cpubenchmark.net/cpu_lookup.php?cpu=AMD+FX-8350+Eight-Core&id=1780 as well as many other performance sites. It's not a 4 vs 8 issue, it's chip architecture. A scattering of PassMark CPU Marks shown below for some procs from which I based my PD user experience
i7 4930, 13605
FX 9590, 10463
i7 3770, 9577
1090T, 5715
x4 965, 4293
i7 4500 , 3880
P8700, 1672
Your current i7-920, 5012, and FX-8350, 9069 are in the range of what I based my comments on. I'd choose the best in the allocated price range.
2) With the AMD's CPU's you don't have access to Intel Quick Sync video encoding within PD, good or bad depends on individual user video editing needs. I use my one box with QS a lot for some encoding tasks, hardly at all for others.
3) Since you may have your box for 4+ yrs, maybe your CPU choice might depending on what CL does with multi-GPGPU support in future releases, or at least hedge. Obviously if you monitor the forums, currently many different user experiences. CL claimed first video editing product with multi-GPGPU support with PD11 release and TrueVelocity3. I'm not necessary talking hardware encoding but transferring some tasks to other processors for enhanced encoding throughput, utilization of OpenCL which PD has migrated to vs previous CUDA(Nvidia) or Avivo(AMD). Will one be able to effective use CPU, integral GPU (Intel), and added GPU card[s] (AMD or Nvidia) processing power with PD over the life of your box, who knows. Other video editing products appear to have an edge here, maybe CL will step it up, CL TrueVelocity4 at PD12 did not appear to be much improvement over PD11 to me.
4) Not just a CPU choice, good subsystems can improve OS performance and PD editing experience significantly.
Jeff