from my 3D adventures - the cpu handles encoding / decompressing / calculation related process - ie - rendering a video to mp4, encoding - or calculating physics in a game - this also depends on the software as some use the cpu for rendering and physics, while others utilize gpu (video card ) for rendering and physics.
now the ram on the gpu is generally what holds textures and calculates the lighting - so your video card is really 2 parts ..a mini computer all it's own ...with a gpu and ram just for graphics. where as your cpu is focused on math, it's not to say that either can't do each others jobs, so part of the performance depends on how the software utilizes them. - hence the idea behind polishing a game - optimizing everything to use all parts of your computer more efficiently.
Ram - is more about how many programs you can start up at the same time, you can have super high performance with just 4 gigs ram - providing you only use one program ...since I do 3D , I often have at least 5-6 programs open, along with my browser - the browser graphics are usually handled by computer ram, where as graphic engines use the graphic card ram
a faster drive allows you to read /write files faster, but once it's up in ram, it only matters for cache speed and virtual ram
to make a long story short, I found that your computer is ultimately as fast as it's slowest part, like if you have a state of the art video card, but your cpu has trouble calculating things like physics, your game / video capture will still stutter.
The other way is true as well, if your cpu is amazing, but your video card is choking - you will have poor performance.
most drives today are fast enough for media performance, so most upgrades on that point should be about storage space, unless you plan to go with an SSD Drive which will help speed up your computer with cache and operating system speed.
4 gigs ram is good for casual internet surfers - 8 gigs ram is fine for average gamers, 16-32 is needed for graphic artists / animators.