Intel i5 3570K or i5 4670K

Only by a small amount.

A game coded to use up to 8 cores will run better on an FX 8320 than on an FX 6300. But because the i5-4670K using 4 cores has almost the multi-threaded performance (about 10-15% lower) of FX 8350, the pure number of cores is irrelevant. The difference between them will be pretty small to the best of my knowledge. Please do explain how having 8 cores will make it somehow perform massively better, if ALL 8 CORES have only slightly more processing power than the i5's 4 cores? I can't see the difference being any greater than the ~15% that the FX 8350 has over the 4670K right now.

For example, i'll represent each processing power with a number (purely for example). Each of the i5's cores have a power of 20. Each of the FX 8350's cores have a power of 11.5. So a game that can use 8 threads, the i5 will have a total power of 80 (20x4), while the 8350 will have a total power of 92 (11.5x8). It makes a huge difference comparing similar architecture. For example comparing the i5 20x4 to a 4930k for something like 20x6. Or for an FX 6300 (11.5x6) to FX 8350 (11.5x8). But the 8350 won't outperform the i5 by a landslide purely because it's only 10-15% behind the FX 8350 using its 4 cores versus the 8350's 8 cores.

Hopefully this doesn't come off as condescending, its just easiest for me to explain what i mean through example. If I'm misunderstanding something or theres something i missed please feel free to correct me, this is just all to the best of my understanding

Also single-threaded performance of a 4670K is almost 40% faster than an 8350 both at stock.

Until You have a game that will use 100% of all 4 cores of an i5, you wont see any real performance boosts if its optimized for an 8 core.

Think of it this way.

say that a game, optimized for 8 cores, Played on an i5, which has 4 cores, uses 60% of the CPU on all cores. That game, on an 8 core, would run all 8 cores at 30%.

Both games still have plenty of room to spare, then 8 core has more. But both games will perform the same, because neither use the CPUs full power.

 

Also, people are getting the whole mantle thing wrong. Or just, looking at it wrong. Sure. It lets you use up to 8 cores, but thats NOT the main reason why its awesome. Its awesome because its a VERY low Level API for GPU access. Which will FURTHER REDUCE CPU usage anyway.

So the point of "8 core optimization" for now, at least, is moot.

The likelihood of everything you're saying coming true doesn't change the fact that until next year it's 100% speculative.  I can't be incorrect before that time.  You can only speculate that I will be incorrect.

Until we have enough multiplatform titles built on enough different engines we can not know for sure how beneficial the new consoles will be in terms of promoting multithreaded optimization or to what degree it will benefit the PC.  The fact that they're using x86-based APUs should help, but we still can't say for sure.  Also, all console games are built on PC and ported to console.

The thing is that with games being optimized for higher thread counts you'd see a bigger bump for the 8350. The difference isn't as big now but will jump with games being threaded for 8 cores specifically. Right now the difference isn't as big because games are also optimized the last gen of consoles. When Ps4 and Xbone one are about 2 years into there life cycle games won't be optimized for lower core counts because consoles won't have low core counts. Even if the  AMD processors have worse core to core performance they will be more optimized for multi threaded tasks. Right now 4 core CPU's are getting a boost from games still being threaded for the old generation consoles. But that will go away leaving better optimization for 8 core cpu's. There will still be some optimization for 4 core but never as much because they will become a minority. 

the z87 chipset is really nice... is it anywhere near upgrading from ivy? no.... I still run an i5 2500K overclocked to 4.5 GHz and it's glorious in Solidworks and MasterCAM... which people recommend an i7 for... Intel is moving towards OC being determined by the chip not the motherboard.... the new chipset is really just a bunch more 6 GB/s SATA slots.... which if you don't have a server and gobs of money to spend on SSD's in raid mode, isn't a big deal

The fact that there will not be any new 8 core cpus for the next 2 years (according to amds own product map) states otherwise. 

I bet something just went wrong with steamrollers production. It was planned for this year and started to kinda come out with the 9370 but was then called bulldozer again. It's possible that AMD is just trying to keep up with the demand of there console APU's. They really are hurting for factories right now. I bet they would make an adjustment in the next year. They just can't afford to be out of the game for that long and they know it. With them releasing the R9 series they are deffinetly trying to stay in the PC market still... 

There will still be 8 core consoles and 8350's none the less.