This is what is important imo, a list of four, I know 
- Processor - the higher the MHz the faster it can process tasks, with applications that take advantage of multi-core systems, multiple cores help.
- RAM - the more you have the better (I think the higher MHz is faster), so opening large images in photoshop or having LOTS of Windows open it will help with, assuming your not bottlenecked elsewhere.
- Graphics card - very important for playing high end games, good for videos, but you don't need such a good graphics card just for playing DVDs or something on the computer.
- Solid State Drive - a much faster alternative to a hard drive, this should speed up everything in your system from loading game textures, booting up & shutting down, opening programs, saving files.. reading files and having a really snappy feel.
So I would say, the Processor, RAM, and then SSD are most important, if gaming is important I would replace the graphics card with the RAM.
Note: I may be wrong about the game textures, but it makes sense, it loads files from the computer faster.
I assume a good SSD will speed up cover flow in the Finder and cover flow history in Safari 4 too.
2nd Note: you need to get a good quality SSD, like the Intel X25-M G2 - some can be duds and not very good