A GTX 660 is actually a decent amount faster than the GTX 460 and HD 5770 listed in Far Cry Primal's minimum requirements, so that wouldn't be a problem at lower resolutions. 1024x768 is also slightly lower resolution than the 720p they were likely targeting for that minimum, so the graphics settings can probably be turned up reasonably high without affecting performance much as well.
As for the e8400, that processor does have a notable thread deficiency compared to the 2-core, 4-thread Core i3-550 and quad-core Phenom II X4 955, but the per-core performance isn't too far behind. Going by Userbench's data, at lightly-threaded tasks, an X4 955 should only be roughly around 15% faster, and an i3-550 around 37% faster (scroll down to single-core speed)...
https://cpu.userbenchmark.com/Compare/Intel-Core2-Duo-E8400-vs-AMD-Phenom-II-X4-955/2720vs2935
https://cpu.userbenchmark.com/Compare/Intel-Core2-Duo-E8400-vs-Intel-Core-i3-550/2720vsm632
And according to this article, Far Cry Primal's performance tends to be limited mostly by the performance of a single processor core...
https://www.dsogaming.com/pc-performance-analyses/far-cry-primal-pc-performance-analysis/
In the screenshots there, you can see that one core is at 100% utilization, while the other cores on their 6-core processor are seeing much lower utilization levels. So, on that two-threaded e8400, the processor might be running that one major thread mostly alone on one core, while grouping the other, less-demanding threads together on the other.
I suspect there might be some performance hitches on that system from time to time, but it wouldn't surprise me if that game were playable while maintaining 30+fps most of the time. As the others said, game requirements generally are more of a guideline than anything, though if some of your hardware doesn't meet the minimums, chances are good that you may encounter some performance issues.