Hi Glen

For yours spec I would say 650W should be good enough and if you have larger then you are OK,I've run 550W PSU with X5675 Overclocked to 4.2GHz easily and I've run 8 HDD plus 2 SSD and two GPU(R9 290X and GTX 560Ti) and with this spec I've never have issue with BSOD etc,later on we are replaced for 750W PSU,but this has been down to the as my friend who bought from me this PC has run two GTX 980Ti

Regarding RAM,if you are hitting actual limit of RAM and then you are swapping the Virtual RAM from HDD then you will and can hit bottleneck which is pretty normal and best way is to upgrade RAM,regarding speed of the RAM,on sever board I would use same RAM and ECC with same speeds,but on normal PC which is OC to moderate speeds,I would go with any RAM,I've run mixed RAM combinations(2X4GB 1333MHz,2X8GB 1600MHz,2X4GB 1600MHz) and never have any issue and my PC has been OC to 4.4GHz and RAM actually run at 1700MHz at 1.65v,,yes is great if you can use same RAM,but sometimes you are listen to marketing stuff which claim mixing RAM is bad,I've run and I'm running now on my PC mixed combination of RAM(4X8GB and 2X2GB ) and PC is stable when I do render with Poser or any other render engine like is V-RAY or Corona Render

Regarding yours scene,you are not alone who likes large scene and in yours case I would say volumetric lighting is killing yours renders and this can be issue with any PC unless you will go route as above seachnasaig render nodes which would speed up yours renders tremendously,I've used few times volumetric lighting in my scenes,but at the end I've abandon that approach,right now I would postwork that in PS or do this render in other faster engine

Good luck there and hope this helps

Thanks,Jura