G-sync, an Nvidia exlusive adaptable synchronization of the monitor to the frame output of the GPU; very relevant to be the same as the monitor if you get one with that feature. G-sync is more expensive than FreeSync from AMD though. More VRAM let you have more detailed textures and graphics. Many games run almost the same on both versions.
I won’t say either option is better than the other but I’ll mention that you could. Large cases usually fit smaller motherboards. Smaller cases however will not fit a larger motherboard and may also not fit the length of a PSU or graphics card or the height of a CPU cooler. Uses FreeSync instead of G-sync which is based on an open standard called VESA Adaptive-Sync which make FreeSync monitors cheaper.
During tests game will be optimized, so by the release system requirements may change, but not that much. Can’t really go i3 or GTX 1060 at that price-point with all those components. 80+ gold supples are more efficient and usually have better capacitors and warranty.
By now though you likely should get the 7-models instead? The price difference I think is around $20 but they are also clocked a bit higher. Maybe with an old motherboard with updated BIOS for support if necessary to save some money there. I don’t know if the motherboards can be upgraded with an USB stick and just a new processor or not. I also don’t know if they are already sold with BIOS versions which support Kaby-lake or not. I don’t know what graphics card you’re going after but if it’s supposed to be able to keep up with the PS4 Pro whatsoever RX 480 or GTX 1060 it is.
And maybe then you’re being a bit cheap on yourself if you took something like the i instead of the i5 6400? You can save some money by getting a H110 or B150 motherboard as said. Similarily if you were thinking of the i5 6600K with Z-motherboard and cooler you can for just a little bit more get the i non-K with H-motherboard and the included cooler.
Ryzen processor cores will be more competitive / comparable with Intel ones than the old FX cores but we don’t know price or exact release date. The older G-processors didn’t had hyper-threading support but these new ones does which make them pretty close to i3s but .. As said it seem like they may lack AVX instructions support. Right now I’d say the G4560 is a better value for such a build. Also stick with H110 motherboard so you go cheap on everything rather than spending more for something else making it a more expensive build anyway without getting the performance of it.
Latest versions have some software for streaming gaming too but you’ll miss out on Ansel if you care about that. I have a very bad AMD Phenom X and there’s lots that can’t handle in a way which feel nice. There’s also titles it can handle but .. With an i3 though I would feel many titles ran good but understand that there are some where it doesn’t run it as well as you’d want. If you have a case which can fit Micro-ATX and ATX motherboards. Already had a HDD and maybe had a PSU you could use then it would be easier to have better performance because you wouldn’t have to pay for that stuff.