As usual, It depends on a lot of things. If you use large textures more RAM will be the best option (it should improve performance of everything you do on your computer). If you use Superfly you could use a better graphics card (so choose the power supply). If you use Firefly a faster processor with more cores would improve render times (but it could be hindered by a small amount of RAM). There isn't a single best answer. It all depends on your promblem. Do a render and check your system statistics. If you run 100% load on your processor with very little I/O the bottleneck is likely the processor. If you see a lot of I/O you are probably short on memory. Most likely you find 100% CPU load and a lot of I/O so again no easy answer. Do consider the generate heat. Extra RAM doesn't generate (a lot) more heat. A bigger power supply and video card could cause heat problems. A bigger CPU is very likely going to cause problems. All things considered I would suggest more RAM but don't expect wonders for rendering times (anything over 20% improvement is a unlikely).