What new piece of hardware to get to increase video rendering performance?
A GPU will give you the best performance boost, but your rendering software has to be optimized to use the GPU; most is.  Upgrading the CPU is the best option if your software doesn't support GPU rendering.  Upgrading RAM won't give you a noticeable performance if you have 8 or more gigabytes.  You might see some slight increase in performance if you have less than 4 gigabytes.

Some notes on upgrading components.  With a CPU, higher clock speed isn't necessarily better.  Unlike gaming, rendering can make use of as many cores as are available, so get a quad-core at least.  Put your money in more cores rather than higher clocks (unless you game on the side).  An Intel Core i7 or Intel Xeon is ideal.  With a GPU, Nvidia is best.  Most rendering software takes advantage of CUDA which is specific to Nvidia GPUs.  There is immense amounts of information about 3D rendering times on different GPUs available online, use that to make your decision.  I recommend the GTX 660 or 760.  Quadro isn't worth the money unless you need weeks of nonstop uptime from your GPU.