The first parts to upgrade depend on what task you use your PC for, however you could start with the RAM, you will need it anyway in a newer setup, then if you are looking for Gaming performance or anything that uses intensive GPU upgrade the GPU, if you need compute power for heavy calculations then upgrade the CPU before the GPU, finally exhaust the full power of your motherboard and when you max out it's potential then upgrade to a newer motherboard, your GPU, CPU and RAM will still be supported by the newer motherboard as you upgraded them to a higher spec, what was the limit of the older motherboard is now the baseline of the new motherboard.
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