...
For 939 with 1 MB L2 (such as FX), when available, it would perform similar to a 940 FX/Opteron, running at the same frequenies.
For 939, if used with unbuffered, non-ECC memory module, can be slightly faster than a 940 due to less memory overhead.
Both of 939/940 dual channel have 80% more effective memory bandwidth than the 754, hence they are always better in performance, especially for memory intensive programs such as video and image streaming, applications using spatially structured data as in scientific computation, up to 20-80% higher performance (e.g. PCmark02 memory test, Sandra memory bandwidth, Sciencemark Stream, many scientific programs). For video, image streaming, data needs to be refreshed constantly from the main memory (L3) to the on chip L2 via the memory bus as size of data >> L2 size at any given time. Under such situation, the high dual channel memory bandwidth delivers a marked performance advantage.
From a few gaming benchmarks, a A64 FX/939 at 2.4 GHz performs 12-20% better than an A64 754 with 1MB L2 at 2.0 GHz, and 15-29% better than an A64 754 with 512 L2 at 2.0 GHz (memory bus, HT bus same frequencies). Not clear if 754 CPU's were clocked to same speed, what would the performance difference be, as the performance difference can be attributed to both memory bandwidth and CPU raw power, but these numbers put an upper bound on gaming performance of 939 over 754. From looking at another set of game benchmarks with both a 939 (512 KB L2) and a 754 (1 MB L2), both running at same frequencies, they are about tie. So I would conclude that for gaming, if both 939 and 754 have the same L2 cache size, running at the same frequencies of CPU, memory and HT, a 939 performs few % (say 5%) on the average better than a 754 for most games.
If not counting memory intensive programs, the advantage of 939/FX over 754 is few % (say 5%) on the average.
...