Download afterburner, or something similar. You may have to go into the options of afterburner to enable monitoring VRAM I can't remember the defaults.
I have a 970 and I honestly don't worry about it. Going over 3.5GB is just like going over the VRAM for any card, in fact the way I understand it, that 500MB slower VRAM buffer is still better than system RAM in that case. Less pulling traffic over the PCI-E bus once it starts acting as a cache and filling up. It can just swap shit back and forth all on the GPU, instead of going to the RAM.
That's true.
I'm assuming the CPU they downclocked was an Intel i5 or i7, which is far more efficient per clock cycle than an AMD, but even so. If either of you do end up seeing some sort of CPU bottleneck, you can always consider an overclock at that point.