What does Dazstudio draw on most when rendering or assembling scenes?
I don't know if this is the right place to ask about computer stuff. But does it draw from CPU, GPU, RAM? I need to update something in my computer. A targetted answer means a targetted purchase ^^ and cheaper.

Comments
The CPU (and/or GPU if using CUDA cores for Iray) willbe used to calculate what colour/brightness, etc., each pixel should be. RAM wil be used to a) hold the details of eeh scene (geometry and materials) (VRAM of graphics card may also nee dot hold data too, if used for Iray).
Do you use 3Delight or Iray? Do you use Windows or MACOS?
iRay and Windows
A GPU upgrade will generally yield the most improvement but without knowing the specs of your existing system you need to keep in mind that otherfacvtors might limit your gains. If you post all of your system specs, CPU model, amount of RAM, motherboard model, power supply wattage and rating, and existing GPU and how much you want to spend, we'll be able to give you better advice
Intel i7 4790 @ 3.60GHz
RAM 8GB (hmm)
NVIDIA Geforce GTX 1050 Ti
I feel if I upgrade, (it is an embarrasingly old machine with a newer graphics card) I will need to replace motherboard too.
You only absolutely have to upgrade motherboards if you also upgrade CPU.
8 Gb of RAM is a little on the small side for DS and you'll need more RAM to go to a GPU with more VRAM. Luckily RDDR3 is very cheap on eBay. You could pick up 16 or 32Gb easily enough.
Before upgrading your GPU you need to open up the case and examine the power supply. There should be a label indicating its capacity, if its 300 watts or less you'll need a new to upgrade the GPU substantially, and then you need to check if the power supply has a PCIE power connector, it should be marked as such either on the connector or on the cables.