Great model lonewolf, stunning detail.
Cdizzle is correct, you just need slightly better scene management. Vray proxies enable you to unload and load models into memory based on the buckets which are rendering. The smaller the buckets in the renderer the more unloading/loading that happens but it will only use available ram.
some more info here:
We render billions of polygons at work at 10-20k resolutions for print using this method. Mostly Vue work when it calls for it but we have 12gb of ram on the machines we are using on the render farm and this is with full GI solutions.
you might also try separating parts of the models into Xrefs and loading them in, this will make your render scene much lighter and give you the ability to turn them on and off when you're not working with the models. It also means you can make updates in isolation and they will pass through to your render file.
Will be great to see this badboy textured!