hey man in answer to all your posts the the two most important things are the cpu and the gpu.if you are into gaming then you are gonna wanna splash out on both. the the graphics card will vastly improve your frame rate or how quickly you can render images in software like premiere or 3ds max.the cpu is the heart of your machine so the more you send on it the better.if you have the money a higher end intel core i7 would be your best bet,if you are on a budget then the amd phenom x4 is pretty sweet of an intel core i5.
another factor in this decision is what you want to use your hardware for.i dont know about the other guys on this forum, but as a vj i am using a macbook with an nvidia 9400 which more or less covers off everything i need.i do produce my own content in flash,after fx and other software,but i do this on my desktop which is fairly old so i'll be upgrading it fairly soon, so i have been through this line of questioning and basically the more you can afford to spend on any piece of hardware the better but if you want an all round machine dont scarifice money on one piece of hardware at the loss of another.
and if you are running a 32 bit system like xp you wont be able to run more than 4 gigs of ram so try get a high end windows 7 or a premium copy of vista.
pcspecialists.co.uk is a good site to build a decent machine for a reasonable price