depends on what you're using the computer for. I would mainly focus on CPU (Core i5, i7) and RAM (around 4 -8GB), but for specific uses, you should spend more on:
Gaming -> Video Card, Sound Card
Music/Video Editing -> Hard Drive, Video Card,
Multimedia -> Hard drive, Sound Card, Video Card
Also, are you starting out new or bringing old components (keyboard, monitor, etc) from your old computer too?