It's a fairly complex question tbh.

There is a lot of info on DAZ Forums for good render speed specs whether for PBR(iRay) or 3Delight renders.

WIth little info to go on I would suggest:

That the cheapest bang for your buck would probably be to invest in some more RAM less than 3GB usable is pretty low. When a render engine runs out of RAM it defaults to virtual memory which will slow things down a lot. More Ram will also speed up the UI if it lags at all and allow for bigger and more complex scenes.

For use of iRay I would suggest looking at a Cuda based Nvidia GPU card.

Hope this helps :)