Buy the biggest hard drive, the fastest processor, and the most VRAM you can reasonably afford. You can't have too much memory. With graphics apps like PS and LR, more is better.
Thanks Nancy, is the processor really that importat for photoshop? or can i handle itself with a thrid gen inter core i5?
This is a judgement call only you can make. Performance varies by workflow and how many bells & whistles you use. A latency of 2 seconds may not seem like much but over the course of a year it adds up to more waiting and less productivity.
See links below.
Personally, I don't buy new computers very often. My current work box is 7 years old and holding steady. Although not super fast by today's standards, it was state of the art when purchased. So I paid more for some longevity. When the time comes to replace this one, I'll do the same thing and buy the very best I can afford -- CPU, Power Source, HD, Memory & Video Card.