Spicing up rendering times (without breaking the bank)

Here's what's wrong with your approach:

Any incremental improvement in your system will cost you far more money than what miniscule real-world improvement in performance justifies.

That is one of the reasons why we strongly recommend an entire new system at this point instead of wasting any of your hard-earned money big time.

Now, if you have to do only a selective upgrade, I'd recommend replacing the CPU, motherboard and RAM all at once. You cannot reuse your existing RAM at all in any new build because it is both physically and electrically incompatible with any newer motherboards.