According to DAZ, Minimum of 8GB Ram and 980+ For large scenes.
Also, Another thing:
If you are mixing IRay and 3Delight materials, Eg:
IRay Hair, 3Delight eyes, Etc. It will cause more strain. And take longer.
If you are using purely IRay, When you render, Ensure that the "NVidia IRay" is toggled in the top right AUX box. As this speeds things up massively. As in, it used to take me an Hour to render, Then when I clicked that, It takes 5-8mins. All in 720-1080.
If you're purely using 3Delight, Adding Lights to the scene actually makes the render faster. Not sure how or why. Something to do with the Pixels.
Enable the use of DAZ to use your CPU and RAM. This will speed thing also.
If you plan on doing a scene with 2+ People in it, and don't have the minimum for your PC to render it quickly. Instead of waiting 2-12 hours for the render too be done. Render each image individually, Then go into photoshop and splice them together.
Hope this helps.